Rails Searchable API Doc
Nicely formatted API docs for Rails, available for both online use and downloadable for local searches(!). Formatting's much better than other sites I've found, and the local search smokes ri.
NOTE: You need to customize the contents if you want Rails and Ruby APIs. I'm using the Rails 2.3.4 + Ruby 1.8 + Nokogiri 1.3.3 for my local dump.