Here’s a demo of a login system built using Ajax. This is a cool implementation which allows a user to login, without having to refresh the page. Here are some advantages of the noted by author of the code:

• User does not need to refresh the page to login.
• User is notified instantly on incorrect username/password combination.
• Overall user experience is more seamless.
• Password is not sent in plain text ever (more secure than traditional system).
• Javascript convenience with server-side security (uses PHP/MySQL).
• Uses one-time use random seed to hash the password before sending (making interceptions useless)

Source code for the login form is also available on the site.

Link: JamesDam.com ≈ AJAX Login System Demo

Here’s a good article on design patterns for RIA.
“Rich Internet applications (RIAs) can provide opportunities to design much better user experiences. They can be faster, more engaging and much more usable. However, this improvement is not without its downside—RIAs are much more difficult to design than the previous generation of page-based applications. The richer interaction requires a better understanding of users and of human-computer interaction (HCI). Although there is a lot of HCI material and research available, it can be difficult to determine how it applies to this new environment.”

Link: Usability for Rich Internet Applications