API

Ajax based online ruby API documentation

http://www.rubybrain.com/

Two best online API/Rails API

Today I found two best API sites. One is http://www.gotapi.com.
This site provides APIs of almost all languages.

The other API site which is only focused on Rails is http://www.railsbrain.com/. I like this most. You can also download this API. It is AJAX based fast, useful.

Update: APIdoc is also very useful. You can get community help here.

Cartographer: A Google maps API in Rails

If you want to insert google maps in your rails application, then you can use cartographer.
This is still in development.