XWii is a program that translates input from the Nintendo WiiTM Remote into X11 input (such as keyboard events and mouse movements).

This means you can play games and control programs in Linux with the Wii Remote, Classic Controller and (soon) the Nun-chuck.

You need to be running Linux with X11 (if you have Linux but don't know if you have X11 or not - you probably do).
You need a Linux-compatible bluetooth dongle or similar device (my belkin USB dongle works great).
You obviously need a Wii Remote