There are a lot of such software on the market. I was also in search for some help desk for my company and I think that these three stood out from the rest:
Zendesk.com | Customer Service Software | Support Ticket System
Orcadesk | Support ticket system | Help desk software
http://www.kayako.com/products/fusion/

We have decided to use orcadesk because it is the cheapest one and has everything we need but you will see which best meets your needs.