With all the work I've been doing with Ruby on Rails recently, I haven't given much thought to the advancements being made for web-based application development. However, yesterday I stumbled across TIBCO's General Interface.
General Interface is an AJAX GUI design framework, with it's IDE also published as a web application. It communicates with the N-tier via XML-based web services and has a very good selection of well written controls (lists, matrices, charts, etc) for embedding in your application.
I've played a little with General Interface and I'm generally blown-away by it's power and ease of use. The only two things stopping me from jumping in and doing some serious development, is the fact that it is only a GUI interaction tool and not a complete development stack. I would still need to write the N-tier somewhere (yes, I know I could publish Ruby on Rails applications as web-services only).
If you're developing web-based content for an existing application (say, Salesforce or NetSuite) and are looking for an easy to use, powerful, web-based GUI framework. I would suggest evaluating TIBCO's General Interface. Or at leaset having a look at some of their screencasts.