quick little lan games: low requirments
armegatron - freeware ton like game
assault cube - very light FPS
battle tanks - little tanks battler
blobbyball - tiny volleyball game
bzflag - better tank battle
counterstrike2d - nice little topdown C-S
cultris - fun tetris game played online or lan vs all
gang garrison2 - awesome 2d platform team fortress 2game!!!
kong - kinda the quake eqiv of CS2D
pocket tanks - nice scorched earth clone for lan
recwar - tiny very fast tank battle
scorched 3d - self explanitory
soldat - really cool platform shooter
teeworlds - very fun kirby w/ guns game
vassal engine - free online board game engine - modules like epic duels rock
warsow - very nice freeware FPS kinda quake like. very fast paced
wormux - worms armageddon like game. good quality
^- all freeware -^

little bit bigger games:
BF1942
star wars battlefront
counter strike
freelancer - demo works over ip
ghost recon
gta2 -now freeware
halo
hardwar - freeware 'space trader'
jedi outcast
magic the gathering
neverwinter nights
savage - freeware like an RTS but plays like RPG.
starcraft
team fortress
tremulus - like savage. people can build buildings then run and fight. also def
unreal tourney - UT demo and UT2k4 demo both awesome.
warcraft