Mac OS X Leopard’s Spotlight search will search through dictionaries you install on your machine and enable. And, it just turns out that such a dictionary exists for Ruby on Rails written by Priit Haamer. Here’s how to set it up:

First, download the Ruby on Rails dictionary.

Next, put it in your ~/Library/Dictionaries folder. If the dictionaries folder doesn’t exist, create it.

Open your Dictionary.app and go to the preferences panel.

Dictionary Preferences Panel.

Finally, check the box next to Ruby on Rails and close the preferences panel.

Now you can search for methods in the Ruby on Rails API by hitting ? + Space and typing the name of the method.