ComputerCraft is a mod which adds computers into the world of Minecraft. It features a multitude of inbuilt programs that are programmable using a simple Lua programming language.
Each computer has its own files, OS, DOS-like interface, comprehensive help system, games, the ability to interact with Redstone signals, and send and receive messages wirelessly with other computers/turtles (with RedNet API).
The OS has a few extra fun programs, like playing Star Wars, and playing "Worm", a pixel version of Snake, as well as a paint program.
More info is available on the the Official Website
You can watch a video that demonstrates most of the new features in Tekkit Lite's version of ComputerCraft here.