Old thread...
Kayako eSupport
Favorite out of all helpdesk scripts, it has every function/feature you will ever need. It's a litle pricy.
Cerberus
Never used it, tried the demo and it looked difficult from a admin's point of view, the front-end is not that great and easy to navigate in my opinion.
Price is okay.
SupportTrio
http://www.activecampaign.com/supporttrio/ Ever heard of it? I just found this a few days ago, and it looks great to me just like eSupport but with a few extra features. Integrates with Modernbill, that is a great feature. Price is not bad.
DeskPro
Great script with a lot of features, but the down side is the price, too expensive for the features it offers. Too expensive.
H2Desk
Found this recently, looks great and simple, but a bit expensive for the features it offers, rather pay a bit more and get many more features.
Those are my opinions on the helpdesk scripts I've had experience with
