Fortunately, Vladimir Kolesnikov has brought us railsapi.com, a searchable Rails API site. It's available to use online or to download and use offline. The search returns matches by relevance and picks up on inexact matches. Below are links to the online API.
Rails version 3: http://railsapi.com/doc/rails-v3.0.3/
Rails version 2: http://railsapi.com/doc/rails-v2.3.8/
EDIT (Feb 2015): railsapi.com is no longer up. But the official Ruby on Rails API site now boasts a clean design with highly responsive search. See it here: http://api.rubyonrails.org/