Lately there's been a bumper crop of JavaScript libraries for creating Ajax applications. At first, it was largely a grassroots effort, and to this day some of the most popular libraries like Prototype, script.aculo.us, Behaviour, Dojo, and MochiKit are maintained mostly by one person.
But now the big boys are joining the party. Yahoo has a modular user interface library that assumes you're doing most of your client-side development in HTML and JavaScript and just need help smoothing out the warts in those languages. Adobe has an HTML-centric framework called Spry. Taking a vastly different approach, the Google Web Toolkit lets you write Ajax applications in Java, and then compile them into HTML and JavaScript. And Microsoft has a library code-named Atlas, which includes both a server-side ASP.NET library that doesn't require JavaScript and a client-side JavaScript library.
The best part: all are available for free, and all but Atlas is open source. (Atlas will have a "reusable modification license," whatever that means, when the final version is released.)
Don't forget Rails and RJS! Write your ajax in Ruby and compile to JavaScript. I tell you, we're taking over the world…
Ah yes, I've been keeping my eyes on Ruby on Rails as well. Its radically simplified development model has inspired countless copycats (and handwringing), and RJS is also intriguing.
But being a Python guy, I'm seriously looking at Django for my next project. It has some similarities to Rails, it is by no means a copycat, and allows you to construct an entire admin interface in a few lines of code.