This feature is in developer preview

Develop rich JavaScript-based user interfaces on Force.com

• Integrate Apex and Visualforce pages with JavaScript libraries

Visualforce now lets methods in Apex controllers to be called via JavaScript using the @RemoteAction annotation. With JavaScript remoting, you can build even more robust functionality in their custom apps. Here's an example of how to annotate a method to enable remoting:

@RemoteAction global static String getItemId(String objectName) {
 /* ... */
}

For a great example and in-depth look, see the blog post Quick Look Into JavaScript Remoting.