When it comes to development tools (bug trackers, issue managers, source code browsers, etc) we've tried them all over the years, but we always felt that we were spending more time wrangling issues in the trackers than we were actually fixing or improving things.
Then we tried the Atlassian suite of products and were instantly hooked; multiple app integration that just works, single sign-on, customizable workflows to define your own business logic, fine grained permissions, automated reporting, and the list just goes on and on...
In fact, we loved it so much that at the end of 2010 FatBox became an official Atlassian partner so that we could more easily bring all these great features to our clients.