We use bugzilla, but it leaves a little to be desired.
(defun foo (x y z)
;; FIXME: foo should not barf if x is nil
and then periodically grep all my sources for "FIXME".how will you prioritise and track them - open up all the files?
The simplicity and approach of FogBugz is fantastic. Really worth a look if you have the cash to afford it. That being said - they do have provide it for free if you only need 2 accounts. See: http://discuss.joelonsoftware.com/default.asp?biz.5.482288
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.
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.
Too all the people using comments in the code and the like, that's very good when you're a 1-2 person team, but when you (hopefully) have more coders, possibly QA people and (god forbid) customers who complain about stuff that doesn't work well, you'd better switch to something that's more manageable.
I also try to write code so small and simple that it approaches the complexity of "Hello World." That way, it's easy to find the bugs.
For small projects it's enough to manage all my tasks, including bugs.
Group/open-source projects: JIRA, which was a nice step up from Sourceforge's built-in tracking.