Did you know that you can simply type "gem server" at your command line to start up a web server on port 8808 that presents all the built-in documentation for your installed gems?
This seems to be a little known fact among Ruby programmers, but worth a lot I reckon. I have rediscovered this feature a couple of times and it's enormously useful, especially when working on the train so having to rely on local documentation. It certainly beats ri for many things.
Oh and that's the new Safari 4 beta, if you're confused by the look of the browser window in this screenshot!