I\'ve used Trac/Subversion before and really like the integration. My current project is using Mercurial for distributed development and it\'d be nice to be able to track issues
Jira integrates using a plugin. Its a great tool.
http://www.atlassian.com
FogBugz has tight integration with Mercurial through their Kiln product.
There is also a plugin to integrate Mercurial with Jira. See the webpage for the plugin.
I just put together a command-line bug tracker called b for Mercurial which, although it's not as powerful as Trac and the like, is exactly what a lot of situations call for. It's best feature is how easy it is to set up - install the Mercurial extension, and all your repos have a bug tracker at their disposal. I find this incredibly useful on smaller projects that I can't/don't want to set up with a fully fledged tracker living on a server somewhere, just hg b
and go.
There's a BugzillaExtension for adding a comment to a Bugzilla bug each time you mention its number.
If you're open to another suggestion, you can try Artemis.
Though I haven't used it yet, it looks easy enough.