It integrates with existing Subversion repositories; for example, you can browse the code in your repositories with Trac (it'll be displayed syntax-highlighted), view diffs between revisions etc. etc. Additionally, you get a wiki (e.g. for project documentation), as well as a built-in bug-tracker a la Bugzilla, all integrated nicely into a single piece of software...
It's Free Software, of course (the license changed from GPL to revised BSD somewhat recently)...
A few words on the installation:
apt-get install trac.
trac-admin /path/to/environment/myproject initenv. You'll be asked where your svn repository resides, what's the name of the project etc.
/path/to/environment/myproject/conf/trac.ini, and change the header logo/URL, the default component/priority/issue-owner and more.
trac-admin /path/to/environment/myproject. Type "help" for um... help.
So far I've set up ca. 7-8 Trac instances for various projects and I'm quite happy with it. While I was at it, I also created a tiny Trac article in the German Wikipedia.
I've stumbled over Lars Wirzenius' article Debian Lessons (Subtitle: Project management lessons from the Debian project) today. The article is from 2000 (updated 2004), but is still very, very relevant nowadays. Here's the table of contents (reading this alone would already help many projects out there, I think):
Make sure to read the whole article. This compilation of tips should prove useful for many community-driven Free Software project out there.