beside trolling....

UFOai - it's a remake of the old X-Com ufo game but it's modernized into awesome, alas it's not finished yet but it's going to be super awesome when it's done -it's open source on source forge so you can compile it (the stable bin is very out of date) and play the first half of the game which is more than enough...

tremulous, also open source play either as a human or an alien and wipe out the other team - great game play, the aliens evolve while the humans buy weapons and armor based on kills, early aliens run on the walls and jump at faces while later aliens get huge and deadly... it's in most linux distros or available as an exe for windows. ( i should point out its based on the quake II engine or something, runs like a dream on any machine but the graphics are somewhat dated)

wesnoth, online or campaign mode stratagy rpg, level up units in battle and out maneuver your enemies - a nice simple strategy game thats easy to play but complex enough to become very taxing between the right players, and with ladder matches you can always find a good match.

when i get my joystick working im going to give vegastrike another go, its a elite style trading game set in the universe that game i cant remember was set in, the one mark hamel (luke sykwalker) played the main character, maverik could be your wingman or that crazy chick that was always sharpening her knife desipite being a space pilot - srsly wtf? you gonna lean out the window an stab them? also the bad guys were cats. great game though so hopefully vegastrike will be almost as awesome for space combat, although the original x-wing game had the best space combat ever, jus' saying.

i tried so many browser games and turn/time based multiplay like trivian but they're all shit.