There's nothing out there that has anything near the full capabilities of something like Drupal, as far as I am aware. You can check out Radiant if you want want just a straightforward CMS. There are also a handful of decent Rails-based forums out there. I guess the question I would ask first is, are you looking for an actual "content" management system or a "community" management system? Also, why Rails? I mean, I love Ruby and Rails is pretty cool stuff, but there's a reason we switched to using ClearSpace for the OMC (we were using the Rails-based Mephisto blog engine alongside an underused MediaWiki install).
