here are some for you...the ones with the * are recommended for lower end machines, while the ones with two asterisks require higher end machines. there are really only two good categories for lanning, strategy and first person shooters.
strategy/'see your little men and tell them what to do' games:
starcraft* (more 'actioney')
warcraft 3 or world of warcraft**
command and conquer games (red alert 2, generals)*
empire earth*
age of empires series (more slow and methodical)*
first person shooters:
counterstrike*
counterstrike source (half life 2 is required I believe)**
unreal tournament *
unreal tournament 2004 **
painkiller*
quake 4 **
battlefield 2** (like said before, you'll want at least 10 people or else the games will be slow)
those are just a few...if you're doing fps (first person shooter) games, then I'd recommend quake 4 and unreal tournament 2004. if you have alot of people, battlefield 2 is great. for strategy games i recommend starcraft and warcraft.