Mega Ruby on Rails Reference
This is a "short" Ruby on Rails reference. It's goal is to give you an overview over the most used functions / methods / classes. It's not a tutorial, but as a handy guide when you already know your way around.
The "InVisible Ruby on Rails Reference" is an excellent resource for all Rails developers. It covers models, controllers, views, HTML helpers, ActionMailer, session configuration, etc.. and it's like the most ultimate, largest 'cheat sheet' for Rails. Lots of examples, lists, and references, all organized really well. This is a keeper.
UPDATE, June 6th! This Rails cheat sheet is also pretty good.