What's a good free bug tracking system that integrates with Mercurial and/or bitbucket?

What about bitbucket itself? ;-) It uses some issue manager integrated.

http://confluence.atlassian.com/display/BITBUCKET/Using+your+Bitbucket+Issue+Tracker

If you are a lone developer you can just sign up for FogBugz startup edition for free: http://www.fogcreek.com/FogBugz/StudentAndStartup.html