Help Desk

Not quite sure what you mean by 'a good help desk software' but I guess an app for tracking issues from report to resolution?
If yes, I've used Remedy and Track-It and call centre apps like Apropos and Cisco Desktop Agent

http://www.readydesk.com. It has far more features than most other help desk software at a fraction of the cost.

Bridgetrak is really worth looking at for helpdesk ticket tracking .
For me it was the best in price-quality-features-stability of work ratio when I had to pick a tool.
I see that it even has a mobile version now.

http://www.helpdesksoftware.com

by Manage Engine hasn't been mentioned yet. It's pretty good with a lower price tag.