A good simple to use system. I integrated it with a java Web Service so program exceptions can create new bug items. No more checking logs for those errors...
Easy to setup and configure. Based on Apache/PHP/MySQL any developer familiar with this trio can easily customize and add any features they'd like to this well written system. I choose it over Bugzilla for the same reasons the author created it. Its much simpler than bugzilla to setup and offers all the functionality you'd expect from a bug tracking system. While our products are windows based. Anthlil Open-Source nature makes it easy to customize and extend it. It took 10 minutes to add "attach screen shot/file to bug report" , we even have our win32 applications post bugs into anthill with an http request to keep us in touch with what's happening in the app. It does the email notifications, workflow ,etc. In summary job well done. highly recommended. Keep in mind you can still run it on windows or unix, just install apache/mysql/php.
An open, cross-platform journaling program.
A scientific plotting package.