Tutsplus.com has a great intro tutorial to Sammy.js, a small JavaScript framework that allows you to write single page web apps, similar to Gmail for example. Because Sammy.js is built on top of jQuery it’s very small, only about 20kb. With this framework you can keep track of your app’s state via hash (#) in the URL, without the need to refresh the site as you go from page to page, and of course it allows you to use the browser’s back button as well.

Check out the full tutorial at net.tutsplus.com.