Halcyon is a JSON Web App Server Framework designed primarily to behave as a service-oriented application. It provides a framework for developing light-weight web applications that communicate via HTTP responding with JSON-encoded data. This is ideal for RIAs et al.
Check out the homepage for more: http://halcyon.rubyforge.org/
You can also check out the RDocs at: http://halcyon.rubyforge.org/doc/