Hey, I play loads of Lan games, my opinions are based on games that are-
Easily bought for cheap, easy to set up for LAN, easy to learn and of course, fun to play with only a few people or loads of people
Quake 3- Can't go wrong here, fast paced FPS action with little thinking required
Vietcong- A choice often overlooked, but fun as hell when you get some good players on
Combat arms- Online only, but free to download and if your friends have epic internet; getting 6 players onto one server on the same team and slaughtering helpless newbies. Most fun I have ever had on a PC.
And the #1 FPS LAN game- Urban terror- Free to download, easy as anything to install and set-up, I had a friend who has never played FPS before jump on this game and have a great time with about 10 other players.
Starcraft- Obvious reasons here
Dawn of War- Best thing about this, if you have Dark Crusade expansion, you don't need the CD and it works for LAN with the same copy :D Excellent fun, Iv'e played this 2 days straight with a friend, never gets old
And the #1 RTS LAN game- Rise of Nations- Thrones and patriots- The version I have (One of those "THAT'S HOT" Ubisoft versions) is just liked DoW, no cd needed ;) I played this game for 6 hours straight on the same map, me and a mate never get tired of this game, no matter how many times we play it we don't get bored. Had a game with 6 players once, 4 vs 2, 4 new players VS 2 VET's, the game was surprisingly difficult for the VET's due to its easy to learn interface and style of gameplay
If your friends and yourself are loaded with cash and all have awesome net- Stop wasting time, buy Left4dead/Team Fortress/Counter Strike, best LAN/Online games ever, there will always be new content and there extremely team based (Exception on CS).
If you want any more suggestions or have questions, feel free to ask