Here's a list of games that were popular at previous LAN parties that I've attended.  Let's just say that I'm used to bouncing between different games, instead of focusing on the same tired titles.

Alien versus Predator 2
-Primal Hunt expansion

Armagetron Advanced

Battlefield 1942
-Desert Combat mod

Battlefield 2
-Point of Existence mod
-US Intervention mod

Battlefield Vietnam

Call of Duty 4: Modern Warfare

Civilization IV

Command & Conquer Generals: Zero Hour
-Retarded mod

Company of Heroes

Counter-Strike (1.6 and Source)

Defcon

F.E.A.R. Combat

Flatout 2

Grand Theft Auto 2

Grand Theft Auto: Vice City
-MultiTheftAuto mod

Grand Theft Auto: San Andreas
-San Andreas Multiplayer mod

Neverwinter Nights
-and mods

Neverwinter Nights 2
-and mods

Quake 4

Serious Sam 2

Soldat

Star Wars: Battlefront

Star Wars: Battlefront 2

Star Wars Jedi Knight: Jedi Academy
-ForceMod III mod

SWAT 4
-The Stetchkov Syndicate expansion

Team Fortress 2

The Ship

Tom Clancy's Ghost Recon Advanced Warfighter

Tom Clancy's Ghost Recon Advanced Warfighter 2

Tom Clancy's Rainbow Six 3: Raven Shield
-Athena Sword expansion
-Iron Wrath expansion

Tom Clancy's Rainbow Six: Lockdown

Tom Clancy's Rainbow Six: Vegas

Tom Clancy's Rainbow Six: Vegas 2

Tom Clancy's Splinter Cell: Chaos Theory

Tom Clancy's Splinter Cell: Double Agent

Tom Clancy's Splinter Cell: Pandora Tomorrow

Trackmania Nations (Forever)

Trackmania United (Forever)

Tribes 2

Typing of the Dead

Unreal Tournament

Unreal Tournament 2004
-Unreal4Ever mod
-Air Buccaneers mod

Unreal Tournament 3

Walkover