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.