Rails 3, Ruby and JQuery documentation into Mac OS Dictionary
Files to download
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 files into /Library/Dictionaries folder.
In some cases after unzipping the files you need to open the preferences dialog and enable the dictionary.
Hope it can be helpful.