Quake 3 or Open Arena come to mind first, but you won't have much of a chance against a PC peripherals. There is Teaworlds as well. Uhh Wargus (Warcraft 2), Boswars. There are many more, but that is all that comes to mind.

EDIT: Forgot to point out Quake 3 and Warcraft 2 aren't free. Open Arena, Teawords and Boswars are however