With Rails API documentation inside Mac OS X dictionary you have system wide access to it. So, in addition to Dictionary.app it is also possible find definition through "Look up in Dictionary" context menu shortcut (works in Safari, Mail.app and any other application that supports it) or from a Dashboard widget for Dictionary.

Rails documentation in your spotlight!
One of the coolest things is to have access to documentation through Spotlight. While coding, just hit ⌘ + Space and start typing the name of the function or class name that you're looking for.Since Spotlight supports finding definitions only from one dictionary at once, you have to move Rails documentation dictionary as the first one in Dictionary.app preferences.
Notice that dictionary search for Spotlight was introduced in Mac OS X Leopard.
What about Ruby documentation?
Download
Installation instructions

Donate!
Feedback
What about Ruby documentation?
Don't worry, i'm already working on it. Probably there will be also a combo-dictionary to contain both Ruby and Ruby on Rails documentation together because of the limitations in Spotlight.
If you're impatient and want to speed up the process, you can consider to donate a buck or two :)
Download
Download Ruby on Rails.dictionary.zip file (1.5MB, works only with 10.5 Leopard) and proceed to installation instructions. Windows and Linux versions are available here.
Installation instructions

Unzip downloaded file into ~/Library/Dictionaries folder if you want to keep this dictionary only for yourself. You may need to create this folder if you haven't installed any dictionaries before.
To make it available for all users, drop the Ruby on Rails.Dictionary folder into /Library/Dictionaries folder.
To make the searching with Spotlight work, open Dictionary.app preferences pane and drag "Ruby on Rails" from the dictionaries list from bottom to the top because only the first one in this list will be searched from Spotlight.
Donate!
I'm confident that using API documentation this way will save you a lot of time but generating and maintaining it will take a lot of mine. If you feel like donating, you can do this right here. If not, i could not be happier when you find it useful.
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