Sabre Airline Solutions has open-sourced their AJAX API (called Rico. I saw some applications build with this API a couple of months ago, at the AJAX summit. I don’t know how easy it is to code with, but I can tell you that it demos _very_ well.

RICO is an example of the enterprise-focussed AJAX api widgetry that is completely focussed on replicating existing client-server applications. The idea is to do this without writing too complex javascript code (by letting the API handle the complexity of the different browsers). This lets you end up with something that you can maintain. If you can do this while having an application with zero install, you get the advantages of the web AND the advantages of client server.
This VB-like stuff is a huge market, and a very different market from the consumer AJAX/RIA market being pursued by most web designers. In fact, the web designer set is a little distainful of attempts to “replicate visual basic” with javascript or flash. But there’s a ton of money in business software, and the interface paradigm of client-server software (with data grids, tree views, and tabs driving 90% of screens) is what most business software relies on. It’s not helpful for building a configurator or an ecommerce shopping cart, but boy would it improve applications like salesforce.com.