Fossil-SCM is a nice distributed SCM, where "SCM" has the original meaning of "Software Configuration Management" and not the new diluted meaning of "Source Code Management".
What this means is that Fossil integrates distributed version control, distributed bug tracking and distributed wiki into one repository. Not one UI, like, say, Trac but one single repository.
So, if you clone a Fossil repository, you do not just get the latest version of the source code plus all its history, like you would get with Git, Mercurial, Bazaar, Monotone, Darcs or any other version control system, you also get the current bug database plus all its history and the current wiki plus all its history.
Fossil is written by D. Richard Hipp, who is not only the author of SQLite but also CVSTrac (the precursor of Trac). So, you know it's gotta be good.
If you want to see an example of Fossil in action, just the follow the link I posted: Fossil is hosted in Fossil itself and the Fossil homepage is actually just the Fossil repository itself.
BTW: even if you don't end up using Fossil, just spend some time learning its concepts. It's a rather brilliant design, and you're probably going to learn something which you can apply even if you are using Trac, Git, Instiki or whatever.