Mouse Guard is a roleplaying game by Luke Crane, based on the comics of the same name. The setting is about a community of mice trying to survive against the many dangers of the wilderness, since mice are small, even heavy rain means floods and animals like snakes or crabs are basically giant monsters to them.
The game focuses on the Mouse Guard, a group of mice that take on the mantle and weapons of guardmice to deliver mail, protect the community, fight against predators and so on. If you've ever read a Redwall book, you should have the basic idea.
The system is similar to Burning Wheel (by the same author), but simplified, so it's been suggested as a good starting game for younger gamers. Despite this superficial simplicity, the system is capable of a lot and could be used to run alternate settings like Jedi guardians with some tweaking.
The exception to this simplicity and ease of use is the character creation, which takes for fucking ever...if you are a brain-damaged monkey.