Red Alert, the original. Ruddy classic lol. The game that got me into huge LAN parties. One time me and 7 friends stayed up for over 40 hours playing it. Sad, but amazing.
Day of Defeat: Source is a regular at my LAN Parties, as is Red Orchestra. Playing a 16vs16 clan game of RO with the 16 of your clan in your house is one of the greatest experiences ever.
COD4 is an obvious one too, great game. CS:S is similar too and also very good at LAN parties.
If you can get hold of a load of 360s and TVs then Halo 3 and Gears 2 make fantastic LAN games, especially Halo 3. Most fun i ever had at a LAN event was a recent Halo 3 one. 32 people, 32 TVs, 32 360s, 32 copies of Halo 3 and 1 house. Was one big mofo of a house lol.