My personal favorite LAN games:
Soldat
For those who don't know about it, Soldat is a side-view shooter where you choose from ten weapons, four sub-weapons, and then throw down! The bot capability makes it a must for me when I LAN, as I absolutely hate playing with the other guys (there's always one guy who bought whatever game we're playing first, and totally owns us at it. No competition = no fun). And surprisingly, for being free, the game is excessively well-made and unexpectedly intense - try a team of four humans versus ten normal bots for a sure-fire sweaty-palms "holy crap!"-filled match. The weapons are finely balanced (although you will learn to fear the chainsaw), the maps are quite ingenious, and... I could go on for days, guys. Try it.
Rise of Nations
Nothing says "I love you guys" like nuking your human ally's sworn enemies back to the Stone Age and saving your collective empire.
Age of Empires III
I seem to be the only person who loves this game to death, but it makes for a great LAN. The home city aspect of the game adds a fascinating new facet to the tried and true "build, gather, train, attack" formula.
Quake II
Why 2, you ask? Because you don't mess with perfection, son.
Guitar Hero
Not so much a LAN game, but I love breaking out "Bark at the Moon" on Expert and showing those kids what's what.