You might also want to consider Hudson. It is designed for use with Java projects although there is a great selection of plug-ins available including support for Ruby and Rake. It has a very useful web interface and supports email notifications as well as many others (like twitter, or giant bear lamps).
The community is also very active and there have been several articles on hooking up Hudson with Selenium which you might be helpful for testing Rails applications on the browser side.
Another one look at is Team City which is free for small projects and teams (including commercial). I really like Team City and have used it before for other projects but currently we're using Mercurial for source control and Team City's support was a little too beta when we were considering it.
I switched from CruiseControl.net to Team City and was absolutely shocked at the improvement. I'm partial to Hudson though because of its similar feature set and very active community.