I came across an awesome tool this morning. Priit Haamer has chunked Ruby on Rails, jQuery, and some of YUI documentation into native Mac OS X dictionary binaries. This lets you search those API docs from Spotlight, TextMate, any application that uses the dictionary app!
I have tested the Ruby on Rails API within TextMate. Hover over any function and hit ctl+cmd+d and a little popup will give you a glimpse at the API docs. Seems like a nice alternative to ACK or the normal TextMate function lookup.
More information and installation instructions are available at Priit's site http://www.priithaamer.com/blog
Here is a nice video of the tool