For UT:
UT2K4 -
1. Just a DM (or team DM?) server on standard map rotation
2. An insta-gib server (practice for the tournament)
UT3 -
1. DM server on TITAN mode (one of the new features of the latest UT3 patch, trust me, its really fun), UT3 has a built in map vote system, so no need to set up map rotation.
2. Either Onslaught or Vehicle CTF - the only problem with these modes is that even on the smaller maps they are only good with a decent amount of players (12+). Tank Crossing is a perfect example of a very small VCTF map that would be good.
Battlefield 2. I know a lot of people prefer COD4, but i still love BF2 ^^. Some of the smaller maps on a rotation would be heaps of fun
Red Alert 3? Idk how you'd be able to do it, but it would be kinda cool if we could have an RA3 lobby or something
You could try Left 4 Dead, but every time it's been tried at previous lans, it's never ended up working

I agree on using a custom map for HL2 DM, all of the stock maps are just waaaaay too small for 32 players, i remember last times comp, if you lived for more than 5 seconds after spawning, you were lucky

Lastly, can we PLEASE have a dynamic (or at least, updated very often) list of game servers currently running and how many players they have on the big screen? I mean, if i looked up and saw say, that there were 12 people in the UT3 server, I'd definatley stop whatever i was doing and hop on
EDIT: An easy way to do this, at least for steam, is to have the steam server browser on the projector computer, and enlarge it so you can see all source games currently running