Many public APIs are wrapped by either a plugin or a gem. The two places I normally look are http://agilewebdevelopment.com/plugins to see what plugins are available and http://rubyforge.org to see what gems are available.
Shameless plug: Recently I put together a gem browser at http://gemmy.calicowebdev.com which uses ferret to full-text search gems based on their names, summaries and descriptions.
