Helpstar
We use it for ~1000 users (and 12 support) and it's pretty good. One download, uses SQL, SQL Express, etc. as it's database, web login or e-mail, etc. Reasonably priced, too. Not 100% customizable (I came from Remedy) but doesn't take a full-time administrator just to keep it going. Support even answers the phone and calls back when they say they will.
Oh, and their web site and support runs on their own product - always a nice touch.
Biting the hand that feeds IT
