Sammy - A jQuery Web Framework
Sammy also makes post action handling simple, gives you access to parameters, JS event binding and triggering, partial template loading, and many other features that you might be likely to find in a robust backend development framework like Ruby on Rails or CakePHP. But Sammy also plays nicely with other frameworks like this; in fact, one of the example applications bundled with the source code demonstrates interacting with a backend Sinatra / CloudKit application.
Best of all, Sammy is well-documented and comes with a number of examples such as the one noted above, which makes it easy to learn and experiment with. So what's next? Think about pairing Sammy up with RESTful data stores like CouchDB, and bypassing the server side framework altogether. Hmmm, possibilities...