I have:
Laser Squad Nemesis (not really free, is shareware and crippled free version)
UFO2000 (remake of UFO/XCom for multiplayer. Great but still beta and on hold)
Any none freeware games would be interesting too, I know of Fallout Tactics, which is unfortunately fatally flawed despite being oh so close to being great.
For my rant on the subject, see
here...

Cheers!
lemmy