I tried bbpress on a site I had once, but it was terribly limited in functionality. There are a few scripts out there called "bridges" that can link a certain software with WordPress so that you can still have the single login functionality (I believe the best one is for phpBB).
If you want something very simple, go with bbpress.
If you want an actual, full-fledged, forum software, go with phpBB and a bridge.