For HelpDesks... I recommend eSupport from Kayako.
I'm going to purchase it soon, it's great if you use it along side with say ModernBill.

True it costs a bomb, near $400 for Kayako eSupport and like $100 for ModernBill - but they make a great pair and also your support tickets wont be so much either!

Plus to have even less, I recommend getting some demo's from DemoDemo.com, I'm going to purchase a load there with all the different skins - meaning I'll have less support tickets, and I wont need to hire so many support staff!