The first ones that come to mind are
Left 4 dead
Left 4 dead 2
Borderlands
If you have a valve game that uses the Source engine, take a look at Fortress Forever - its a free team fortress modification.
If you like RPG style games, there is Titan Quest with Immortal Throne expansion - its a Diablo II clone. Then there is the ever popular Diablo I and Diablo II. The original Diablo only supports 4 online players though
If you like deathmatch style games, there is Unreal Tournament 3 Black Edition, Quake, Quake II and Quake III arena
If you have a bunch of friends, I would take a serious look at left 4 dead 2. Its a team based game with up to 4 people on each team - for a total of 8.