Computer Craft

Computer Craft is a mod that does what it says on the tin, it adds computers, monitors, robots (turtles), floppy disks, and more to the game. You can run programs on it, and create your own using incredibly easy Lua programming language. Users from the forum have designed incredibly complex programs, such as a facebook-like internet using wireless modems, and variable sized tunnel drillers. Computer Craft has a seperate wiki here.

Turtles
Turtles are small robots that do the hard work for you. They, too, accept Lua commands. There are several kinds of turtle that can perform different tasks. Meelee Turtle, Digging Turtle, Crafty Turtle, and many more. Turtles require fuel to be able to move, and any furnace fuel is accepted, as well as a few other external mod fuels.

Mod Compatibility
Computer Craft is very compatible with other mods. You can transmit redstone through Red Power bundled cables, and do several things with IC2.