CruiseControl.rb: Continuous Integration for Ruby (and Rails)
CruiseControl.rb is a continuous integration tool written in, and for, Ruby. It makes it easy to set up automated builds of your software and to find out about tests that break on the fly. The official site has downloads, a 5 minute screencast, and a working demo of the tool.
Alongside this, Bill Eisenhauer - the developer of GeoKit, the Rails geocoding plugin - has written an article talking about CruiseControl.rb and showing how to set up custom tasks to use CruiseControl with Capistrano.