A Steam controller is a controller made by Steam, to help with their goal of getting into more living rooms (big picture mode, Steam boxes running a version of Linux optimized for gaming, etc). Its main features are a couple of touch pads, the ability to emulate mouse and keyboard functions (initial previews showed it being used to play things like RTS games, which don't work well with normal controllers) and highly configurable settings.

I was seriously considering getting one, and (after playing D:OS EE), replaying D2:DC as an unarmed warrior.