What system do you use for bug tracking?

I've used trac and mantis. Trac is kinda a pain to set up. Mantis is easy to set up, but very ugly.

Mantis. Not great, but good enough - for now. May replace it if our needs get more complex.

Trac is very nice, as other have stated. It's kinda like SourceForge done right, in the sense that it has source browsing, a wiki for web pages, and a bug tracker with good notification capabilities. Last time I used it it was absolutely retarded about user accounts and saving settings (you had to carry around an MD5 "key", rather than being able to log in).

We're using FlySpray, because it drops into our CMS (Joomla) with relative ease, and we wanted customers to be able to file bugs. We also use it for support. It works pretty well, has all of the features we needed, and is a really nice clean and simple codebase, so customizations are relatively easy. It's also a low-dependency PHP project, so it'll drop onto any ol' hosting account with a database, and runs plenty fast.

I use a heavily modified version of Case Tracker, a group of addon modules for the Drupal CMS. It's similar to other helpdesks (it's actually a fork of the project and project release modules that are used at drupal.org) and it's being extended by a student during this year's Google Summer of Code.

Since Case Tracker runs in Drupal it's just one feature of an entire suite. I have all the features I need, from web-to-email, email-to-web (posting by email or phone), XML-RPC, a full range of access controls, wiki functionality, RSS feeds, etc.

I Second that. Extremely easy to set up and comes with quite reasonable default configuration.