Ratling
Often derided for resembling 'Hobbits in Space', Ratlings have swapped magic rings for sniper rifles, all the better to serve in His Most Glorious Imperial Majesty's Imperial Guard. The friendly wizard Gandalf has been replaced by an Aspiring Sorcerer with a thirst for pint-sized auxilia; thankfully, Ratlings are excellent at both self-preservation and cookery, presumably fending off their enemies with a massed barrage of just-baked bread rolls. This is only to be expected, since 40K began as Warhammer Fantasy IN SPACE! and thusly Ratlings are basically Warhammer Halflings with lasrifles.
Physical appearance
Ratlings are a breed of abhuman, putting them in the same dropship as Ogryns and their not-to-be-named brethren, and they are about as popular with the fanbase as they would be with the citizens of the Imperium. They measure three feet in height, 50% of which is stomach - they can eat twice the ration allowance of a regular Guardsman, presumably in a first, second, and third breakfast. Unlike their fantasy counterparts they disdain adventure, displaying an eagerness to get into cover that in any other squad would be labelled as 'shameful' but for these waist-high warriors should be referred to as 'characterful'. While sociable creatures, make sure you don't leave anything precious lying around, as Ratlings tend to loot anything that isn't nailed down (and a few things that are (and the nails)). Another interesting thing to note about Ratlings is that they breed like there is no tomorrow. Possibly because there is not.
In the game
Reasons of character are about the only ones you would take these diminutive death-dealers. With a mediocre Leadership and limited survivability, Mount Doom will be the place you are headed if you put too much (or, indeed, any) faith into them. They might be the unofficial front for dealing stolen weapons to veteran Guardsmen, but veteran players should move on without a second thought.
What the fuck was the guy above me smoking? Ratlings make a good addition to blob based lists. With BS 4 snipers and free camo cloaks, though 7th ed Sniper weapons lost the true rending ability against vehicles they still function at AP2 on 6s to-wound, so Ratlings can piss off smaller high value units or monstrous creatres and free up that extra shot for your Leman Russ to Pie-plate some Grey faggots. Throw something to take the shit or put them in a building and for just 100 points, watch 10 furry midgets glance shit to death. Oh and I almost forgot; Ratlings are one of the worst units for assaulting with, so keep anything that could charge them very far away or see Frodo be stabbed to death (that could be quite cool).