UnitRecord is a gem that allows you to run unit tests without hitting the database. On the project I'm currently working on, this enables my team to run ~3,000 tests in ~11.5 seconds.
I'm not going to re-post details from my blog or from the documentation, but if you're interested, here are the links:
Details: Rails: UnitRecord - Test without the Database - Dan Manges's Blog Documentation: http://unit-test-ar.rubyforge.org/
Dan Manges