RTS LAN:
AOE2 is a classic, great 8 player fun. Nothing can really beat this for RTS Lan play. Pain to lan sometimes though, everyone needs to have the same OS, sometimes crashes randomly too with a lot of people.
Warhammer 40k Original- great game. Very addicting. Unique races to choose from.
FPS:
CS- with at least 6 people this is a blast.
Serious Sam
BF2
L4D2
Action/RPG:
Jedi Knight 2 Jedi Outcast- with bots, custom mods/maps, this could be some of the best fun ever had.
Nox- absolute Westwood classic. Great hack n slash gameplay
Diablo series
Baldurs Gate 2
Neverwinter Nights 2