For work I have been using GWT/SmartGWT. For fun: Seaside and Pharo
For side projects that require a web UI I like using both Play! and Rails (and a lot of Rails development work in the last 3 or 4 years).
Just recently as another side learning project I have been revisiting the Seaside continuation based web framework for Smalltalk. This week I bought the PDF version of "Dynamic Web Development with Seaside" and when I get bits of free time I have really been enjoying reading through the book with a Pharo Smalltalk Seaside image open next to the book to work along and experiment as I read. You can read through this book online also if you want to check out Seaside.
Seaside provides a very agile development environment, both editing code in a Smalltalk browser and hooks in web apps themselves.