My favourite bugtracker Bugzilla has just released version 3.0.
New features include custom fields, an XML-RPC interface, per-project permissions, better UTF-8 support and much more.
I think the reason that I like Bugzilla so much is that it is a developer's bugtracker. it is not the prettiest or simplest of the bugtrackers out there, but it has masses of useful features and is very configurable. It really does make a developer's life easier since I find it more centred around getting the bugs done and dusted rather than attempting to make it look nice for non-techies.