Official site

• http://www.fogcreek.com/FogBUGZ/

Comments:

• We (Songkick) only use this for user feedback, and we're looking to switch to a combination of Get Satisfaction and ZenDesk. FogBugz is not flexible enough for tagging feedback and has no analytics.
• Playfire use this for issue tracking and its wiki and are considering trying out the estimation tools. We run it locally for maximum responsiveness (an issue tracking system has to be extremely responsive otherwise it doesn't get used).

Start-ups using:

• Songkick, Playfire