UFO: Alien Invasion
Multiplayer is supported over LAN and internet, with head-to-head sessions, but also cooperative games with a maximum of 6 teams playing, against each other, or AI-controlled opponents. The game is free software, licensed under the GNU General Public License.
Martin Gerhardy' (mattn; tlh2000)
' (Arisian), Forest Hale' (LordHavoc; Darkplaces [http://icculus.org/twilight/darkplaces/])