When we have LANs the games that tend to get thrown around for multiplayer are:
Age of Empires 2 - be careful, as over LAN the game has issues with multiple operating systems and Windows 7 in general. We find that if everyone is using XP it runs fine.
Battlefield 1942 Free Demo
These games are either cheap/ free and still gives everyone a good time.
Also when I host LANs there tend to be about 10 people so there is enough to go around and a lot of the time people will splinter and play random games online or go on a console or 2.