I'm currently using GNUe DCL and it's working out pretty well for me. We also use it at my day job (team environment) and it's pretty good. Can be a little hard to get up and running and is a bit rough in places. Used fogbugz at previous job and I'd take DCL over it - fogbugz is not too shabby though.

How about Mantis? http://www.mantisbt.org/

Demo: http://demo.mantisbt.org/my_view_page.php

Mantis is an improvement over Bugzilla, for sure. However we saw huge performance problems once we hit the 50k issue level. It was also much more difficult to modify than Jira (which we moved to). Although I guess among free options Trac and Mantis would be my top two.

Mantis is okay - we used it for a (school) project last year, but for what we needed it for it was overkill.

I use a plain text file.

Jira is fantastic, but not open source (although you do get a copy of the source when you buy it).