Grape is a REST-like API micro-framework for Ruby. It’s designed to run on Rack or complement existing web application frameworks such as Rails and Sinatra by providing a simple DSL to easily develop RESTful APIs.
Here is a nice slide share of the API possibilities (though it is from 2011).
Rack::Cors provides support for Cross-Origin Resource Sharing (CORS) for Rack compatible web applications. The CORS spec allows web applications to make cross domain AJAX calls without using workarounds such as JSONP.