http://bestpractical.com/rt/ -- Kick ass bug tracker for people
that have entreprise issue-tracking needs. I've implemented this a few
times with good results. It's a nice system, it used to be bit of a
pain to install.
http://roundup.sourceforge.net/ -- Very intresting software. It
comes as a relatively simple bug tracker, but is actually more of a
framework to build issue/case-tracking solutions. I wonder how it would
perform as a non-document-centric case management system.I currently use this privately, it's a 15 minute install+configure.
http://www.fogcreek.com/FogBugz/ -- Havn't used this one but I saw the demo and it certainly does some intresting things. It's more a project management tool then it is a "issue tracker".