Probably you find sthg here:
http://en.wikipedia.org/wiki/Comparison_of_issue_tracking_sy...
http://en.wikipedia.org/wiki/Comparison_of_issue_tracking_sy...
The amount of information there is pretty overwhelming, and it seems like this should be a well-solved problem for startups who don't yet have a support person. The main problem is that engineers don't do much support, and aren't accustomed/are too lazy to login to a support tool to communicate through it.
We have some experience with Bugzilla and Roundup. The latter suited our needs better (but they evolved over time, so a direct comparison doesn't necessarily make sense)