Trac by Edgewall Software is at its most simplistic level an issue tracker that provides source control integration (mainly for Subversion). But it is much more than that, it also has a built-in wiki, source browser, and project management through the ticketing system.
It's a really decent tool for make developer's lives much easier and it looks nice too. As you start doing more with it you can really see the power in there. You can work out a roadmap/timeline from your tickets and also generate changelogs.
But all this isn'y new to the world out there, what I discovered today was Trac Hacks which is a repository of Trac add-ins and enhancements. There is a code review plug-in, a blog plug-in, and simply too many others to list.
So combine Subversion with Trac for a good experience, then add plug-ins to fill the holes in the development process.