Dragonmark
When Keith Baker created the Dungeons & Dragons 3e setting of Eberron, one of the things he wanted to do was to explore the idea of "wide magic"; widely available, but not very powerful, magic. This would justify his setting's dungeonpunk theme and its magitek by way of industrialized magic motif. To do this, he invented two things. The first was the Magewright NPC class. The other was the concept of Dragonmarks.
Connected in some mysterious way to the Draconic Prophecy, Dragonmarks are mystical symbols that appear on an individual's skin, and each of which bestows innate magical abilities based on that mark. Kind of like a birthmark that gives you a quick dip in Sorcerer for free. There are a dozen different Dragonmarks, and each mark comes in one of several levels of complexity; more developed dragonmarks can produce more powerful effects. There was a 13th Dragonmark, the Mark of Death, but it's gone now. There are also Aberrant Dragonmarks - see below.
The Dragonmarks are tied to specific races, and more specifically to certain bloodlines. These gave rise to the Dragonmarked Houses; one half fantastical megacorp, one half aristocratic family. Through centuries of work, they have both monopolized their mark, binding together all examples of their mark into a single sprawling family tree, and exploited their mark's power to give them an unshakeable place in Khorvaire's functioning, their usefulness allowing them to transcend mere nations as entities of power.
Incidentally, in 4th edition, it was possible to defy the rules of 3e and have a functioning dragonmark outside of the one(s) associated with your race, but you were told upright that doing this would Get You Noticed, and the DM had every justification and incentive to have people take note of this very exceptional trait of your PC and respond accordingly.
Mechanics for dragonmarks have varied across editions.
The Mark of Detection
The Mark of Finding
The Mark of Handling
The Mark of Healing
The Mark of Hospitality
The Mark of Making
The Mark of Passage
The Mark of Scribing
The Mark of Sentinel
The Mark of Shadow
The Mark of Storm
The Mark of Warding
The Mark of Death
This is the Lost Dragonmark, destroyed when the elf family that held it was wiped out by angry dragons for having created a Half-Dragon Elf. Ironically, that hybrid survived, by becoming a lich, and went on to found the Blood of Vol. Even more ironically, in her undead state, she can't use the powers of her mark anymore.
Aberrant Dragonmarks
So, the Dragonmarked Houses have the market and the potential for Dragonmarks all sewn up, right? Wrong. Every so often, a person of any race is born with a Dragonmark that doesn't quite match up with the others. It has unique powers, sometimes quite potent, but it also tends to have strange side-effects. It's possible that, given time, an Aberrant Dragonmark could stabilize and become a new Mark; the 12 Dragonmarks may owe their existence to interbreeding between the Aberrant-Marked and stabilization. However, Khorvaire society demonizes those who bear Aberrant Dragonmarks; after all, the Dragonmarked Houses don't need competition.