There is also rt, which is built on Mason. It has a significant following. I've never used any of these for more than a few minutes, but I suspect they all work for most people.
I've been using RT for more than one year. It is way better than Bugzilla. Some pros that come to my mind:
Very flexiable ACLs. You can configure easily who can see, reply, comment and admin tickets and ticket queues.
Unlike Bugzilla most functionality is configurable via web interface.
Unlike Bugzilla it is designed to be extandable via add-on modules.
It is written in much better Perl. I'm sorry, probably Mozilla developers are good C++ developers but their Perl is crap (at least it was crap year ago when I looked on it last time).
Not sure how RT compares to PerlDesk as I have never used it but I urge you to look on RT too. It is very good.
Update: I forgot about another pro: RT has two-way email gateway. Unlike Bugzilla you don't just get notifications for new posts and comments but you can reply on them using your faivorite email client.