InfoWorld’s Peter Wayner wrote an good overview of the most popular open-source toolkits, including a helpful screencast about each. Specifically, he covers:

• Dojo (screencast)
• Google Web Toolkit (GWT) (screencast)
• Microsoft Atlas (screencast)
• Rico (screencast)
• Yahoo! User Interface Library (YUI) (screencast)
• Zimbra Kabuki Ajax Toolkit (screencast)

Which to use? Well, GWT is dependent on Java, which makes it a non-starter for everyone I know. Atlas (not surprisingly?) doesn’t play well with others — there are Firefox or Safari compability issues, and although the client part is technology-agnostic, the server part is ASP.NET. Rico is neat, but less complete. Kabuki looks interesting, but it’s also missing interesting features.

That leaves us with Dojo and YUI. You can’t go wrong with either, and any time you spend learning them now will pay off for years to come. I believe these two toolkits will evolve into the dominant frameworks for standards-based client development.

I personally use and recommend YUI because of its feature set, it’s excellent documentation (including Web 2.0 UI patterns), and the excellent support you can get directly from its developers on via the Yahoo! JavaScript Developer Group mailing list.

• InfoWorld: Surveying open-source AJAX toolkits