Although you can run
gem server to have a rails rdoc running on localhost, the rdoc rails document could be troublesome to find a specific method you would like to know about. Fortunately, you may want to try out an ajax improved rails documentation:
RailsBrain provides a free downloadable (free) offline documentation for rails. It has the lastest version 2.2.2 available for download. The ajax feature allows developer search method, class or file name via a input box. It's a sufficient tool and I am using it now for development.
Gotapi is an online API collections. you can manage/view/search all the API you are currently using in any languages(e.g. PHP, Java, .net and of course rails). Similar features are privided as RailsBrains, but it's only available online.
Railsapi.com is a recent new project and is hosted on GitHub. It has better search capabilities. Intresting to note that it does not support Internet Explorer is currently (may be more specific version or not been IE tested). Apart from that, it's smart, tidy and downloadable Rails doc. (Added on March 24 2009)
rails.raaum.org Taylor suggested a very compact Rails documentation site. It's actually a site that lists some useful information for developers including good podcasts, links and posts. So check this out see if you can find any good topics.(Added on July 21 2010)