Celestia
The Seven Mounting Heavens of Celestia is the Lawful Good plane in the Great Wheel cosmology. It is a single mountain surrounded by an ocean of holy water which stretches out into all directions and can be seen from everywhere on the plane by looking down the mountain. The plane is populated by several types of Lawful Good creatures, such as Solars, Aasimar, Archons, Devas, and a number of good-aligned gods such as Bahamut, Moradin and Yondalla alongside their followers. It is pretty much the Christian idea of what heaven is like, and those familiar with the third part of Dante's Divine Comedy, Paradiso, will be feeling somewhat at home.
The plane is ruled by the Celestial Hebdomad, a council of the seven most powerful archons in existence. Aside from their most powerful member, Zaphkiel, all founding members have either perished or passed into the realm of Chronias. These archons were replaced by the most powerful amongst their ranks, meaning that there are always seven members of the Hebdomad. They do not consider themselves as deities or even rulers, rather they are guides, examples and patrons of those who wish to learn. They all resemble their layer's namesake: the ruler of the Silver Heaven is silver-colored, the Pearly Heaven's protector is pearl-colored and so on.
Layers of Celestia
In order to go from one layer to the next you have to scale that section of the mountain. Of course the mountain is a metaphor, as there are multiple paths but only someone who is "truly just and good at heart" can ascend the mountain. Naturally, this means that the DM can deny you access to any layer above the first by citing that one puppy you kicked or that time that you stabbed a villager back when your character was dominated by a vengeful wizard. Of course he could be even more of a troll and let you ascend to the seventh layer, Chronias, from which there is no return; you either transcend and become part of the plane or you did a bad thing once and whatever's up there murders your soul forever and erases you from the multiverse. While climbing the mountain normally is only possible by reaching certain levels of enlightenment, a crafty planeswalker can find shortcuts between the various layers by using portals.
Lunia
The first layer of Celestia is the Silver Heaven, Lunia. If you arrive on the plane you end up waist-deep in the Silver Sea, an ocean connecting to the River Oceanus and its various bodies of water. The Silver Sea is a salt-free body of holy water that all manner of life thrives in, from the smallest of fish to the most gargantuan of leviathans. Because the Silver Sea is made of holy water, evildoers who end up on the plane will receive horrifying burns because of the stuff, making a perfect first line of defense against any incursions.
Lunia is cloaked in perpetual night, but because of its many bright stars the layer has a silvery shine to it, making the dark not quite, well, dark. Lunia sees frequent trade with the sea elves of the material plane, as well as various pilgrims to the realms of Tyr and various ocean-gods.
An unusual point of interest in Lunia is Castle Mahlhevik. Owned by the formerly chaotic evil wizard of the same name, Mahlhevik is set on learning the ways of goodness. After conferencing with high-ranking archons and revealing his sincerity he was allowed to build a castle on Lunia. He's still got a long way to go and his buddies with delightful names like the Soul Reaver and the Lifeleech don't drop by very often (because the Silver Sea might very well kill them), but his castle is a good place to stay if you don't want to deal with the archons.
Lunia itself is protected by Barachiel the Messenger and his trumpet archons. He has a silver skin and silver-feathered wings, wears violet ropes adorned with stars just like Lunia's night sky and carries an electric longsword into battle. He commands the plane's defenses against hostile incursions, but most of the time this means that he just sits and watches as fiend are being devoured by max hit die celestial octopi.
Mercuria
Mecuria is also known as the Golden Heaven, named after the golden light suffusing everything. Streams flow fast, valleys are lush, slopes are tame and it is a place of peace and rest. Alongside the plentiful hamlets on the layer it is also the home of many tombs, monuments and other buildings remember the glorious dead and their valorous deeds. While robbing these tombs might net one very nice treasure this is a Bad Idea; aside from the inhabitants of Mercuria who would tear you a new one for doing this the tombs are often guarded by their own inhabitants. The Deathless can return their spirits from the peace of Celestia back into their bodies, reanimating them to wreck whoever disturbs their rest before laying their bodies back down and return their spirit.
Bahamut's palace travels at his will between the lowest four layers of Celestia, making for an easy way to travel between the layers if one can convice the Platinum Dragon to take them along for the ride. The palace itself it the dragon's hoard, with the windows being gemstones set in precious metals, the floors and walls decorated with the most exquisite marble and statues made from the most exotic materials. Stealing from the Platinum Dragon however is a Very Bad Idea, as will attest the bones of thousands of failed thieves. Other realms on Mercuria are Rao's Sweet Reason and Torm's Trueheart.
Domiel the Mercy-Bringer is the guardian of Mercuria. He is a 12' golden human with matching wings, a flaming greatsword and no clothes. His duties are twofold: prevent the desecration of the tombs of Mercuria and overthrow tyrants and their followers.
Venya
The Pearly Heaven is made of rounded slopes, terraced fields and well-tended forests are common on the layer. Venya is colder than the previous two layers, and is as such frequently covered in snow. When the snow melts it flows down, sometimes forming in great lakes of icy water. One of these lakes is The Glass Tarn, a smooth body of water that will grand one a prophecy if a person throws something of real value to them into the water. But if the offering is not sincere a powerful archon comes out of the water to wreck face.
Venya is the home to two divine realms: the Green Fields of Yondalla where crops never fail, the weather is mild and the harvest is plentiful. Life is rustic and perfect for the gregarious halflings living in the realm, with the biggest threat being investations of moles, rabbits or badgers. Heironeous also has his realm here, The Fields of Glory.
Venya's protector is the handsome pearl-colored Erathaol the Seer. He prefers peace over violence but if needed will go into battle with a quarterstaff part coated in frost and part in fire. He is the patron of seers and prophets and can predict planar events before they happen, and watches over children destined for greatness. He lives in a great undersea library made of seashells and pearl, which houses more knowledge than one could read in a thousand lifetimes.
Solania
The Crystal Heaven is covered in foggy valleys. Between the mountaintops rest massive glaciers whose meltwater flows down into the valleys and into Venya. Solania is home to a massive number of monasteries, cathedrals and other holy places. These places are often the destination of interplanar pilgrims looking for the answers to the questions of life, the universe and everything.
Many deities make their home on Solania. The foremost is Moradin who lives with his wife Berronar Truesilver in the great fortress of Erackinor. Ther armories of the lord of the dwarves are second to none, and the stonework from which the halls themselves are made are peerless. It also houses the Soul Forges, ill-defined magical items whom are said to be used by Moradin to temper the hearts of his people. Other deities on Solania include Delleb and his Great Library, Fortubo's Empyrian Delve and a few others. Finally, Solana holds the realm of Uroboros, the place where Jazirian has her body and combats the machinations of her brother Ahriman.
Solania's member of the Hebdomad does not live in some great citadel. Instead she wanders the layer and helps out travelers in need. Pistis Sophia the Ascetic is a feminine figure whose skin, wings and hair are all crystal in color. She abhors lies and believes that truth holds harmony and bliss. She will always speak the indilluded truth for this reason and encourages others to cast off their possessions and clothes to do away with their greed and insecurity. So if you are ever on Solania and see a large mob of naked people, you can be sure that Pistis Sophia is nearby.
Mertion
The Platinum Heaven is the mustering ground for the armies of good, where large numbers of Good creature gather to take the fight to evil. The plane is covered in large sweeping plains to hold these forces, and hold many citadels and domes with the facilities to arm and supply these forces. One of the biggest cities is Empyria, the City of Tempered Souls. It sits on the edge of a mountain lake and has many healing fountains. These can cure almost anything from withered limbs, lost senses, insanity and life itself. If a fountain cannot heal it one of the many healers and hospitals within the city is sure to help a pilgrim or at least point them in the right direction.
The god of time Cyndor has one of his realms on this layer: Rempha, the City of the Sands of Time. Al'Akbar lives in Soqed Hezi, the City of Swords.
Raziel the Crusader is the most warlike of the Hebdomad, inspiring and guiding Paladins and fighting against tyranny and oppression from the back of his celestial charger, Temperion. The Firestar (as his nickname is) does have a gentle side to him, and when not fighting against evil he can be found in Empyria tending to the sick and guiding clerics.
Jovar
The Glittering Heaven is strewn with twinkling gemtones, a beautiful sight that will take the breath of everyone who sees it for the first time. The fields are littered with archons in deeep contemplation regarding their lives and actions. The primary location on the plane is Yetsira, the Heavenly City. It is a massive seven-layered ziggurat adorned with extremely valuable gemstones. On its first level sits the Exchequer of Souls, a black marble building where powerful archons judge the virtues of the lower ones and elevate them in rank if they are worthy. On the fourth terrace sits the Radiant Arsenal, a long and narrow building with tall ceilings and deep cellars. Here the arms of the archons are stored if the portal to the seventh layer has to be defended against an incursion.
The sixth layer holds the fortress called Pax Exaltea, where warden archons receive new assignments and armies will be lead in case of an attack on Jovar. The fortress' steward is Sealtiel the Defender, an ebon-skinned and hairless archon with massive golden wings and a glistering aura. When he is not tending to the city's defenses he helps other celestials on their paths to enlightenment.
Chronias
At the very top of Yetsira stands a single solar named Xerona. He guards the Bridge of al-Sihal which is the only way into Chronias, the seventh layer of Celestia. Only the most worthy and enlightened are allowed passage into the Illuminated Heaven, from which there is no return: those who are worthy have their essence joins Celestia itself and become part of its spirit. Unworthy ones however will have their souls extinguished and their essence erased from the Multiverse. As such, nobody knows what Chronias is like but most works agree on it being a realm of pure light and goodness.
Zaphkiel the Watcher is the leader of the Celestial Hebdomad, the only original of the group and one of the most powerful beings in the Multiverse. A 10' figure of incandescent gold with platinum wings, Zaphkiel is a sight to behold. He advices the other members of the Hebdomad regarding dealing with mortals and other celestial beings. He is the protector of the spirits of stillborn babies and sacrificed children. He is something of an enigma though: while he has interest in advancing less-perfect beings the reason why is unknown to all others, even gods and the members of the Hebdomad. They are also the only ones to have ever actually seen Zaphkiel: doing so will either kill the unworthy or ascend the righteous into a state of goodness that merges them with the essence of Celestia itself.
In 4e
Celestia appears in 4e as well. Here it is a realm consisting of seven great peaks where certain good-aligned gods live such as Bahamut, Moradin and Kord. Celestia serves as the Lawful Good counterpart to Hestavar's regular Good alignment. It is where the Lawful Good go after they die and live amongst the righteous.
Inhabitants
Celestial archons are the most common inhabitants of Celestia, but angels also appear frequently. On the lowest layer elves are somwhat common, and the fourth layer has a significant portion of dwarven petitioners. Petitioners are all Lawful Good and are turned into lantern archons upon their awakening on the plane. This grants them a few poewrs including being able to cast rays of light, teleportation, the ability to speak every language and immunity to petrification and electicity amongst a few others.