I just used osTicket and Hesk, I found that osTicket has powerful settings more than Hesk. I can't find a way to change email settings including SMTP settings in Hesk.
I see several sites using Get Satisfaction and it appears to be a very good system. Not sure if it is free, but you should at least check it out. I was very impressed with it when I sent support tickets.
osTicket really is easy to install and configure, main advantage of osTicket that I like is No user account or registration required for users to submit ticket.
My vote will also go to hesk for a free one, but for a paid one you might also want to consider LA desk, which is 99$ (one time) per agent and includes live chat and more powerful features than hesk.