SWAT 4 and/or Unreal Tounament, UT 2k4.
SWAT 4 is fantastic for LAN as you can play the entire campaign co-op with up to 16 or so people, it's much mor fun with human rather bot based squads. The comp multiplayer isn't bad either, VIP on that game rocks hard, plus it never gets old perfoming a non lethal takedown on your enemy and hearing the player's cries of anguish at getting tied up and losing 5 points.
The last LAN I went to we had it as a 'just in case' game, but ended up playing it for about 18hour of absolute hysteria.
Unreal Tournament is simple and frantic, 2K4 a bit more complicated.
for RTS games, STARCRAFT! As well as Company of Heroes and Dawn of War 1. They're great.