I am working on the Jaffa LGPLed project
Jaffa (a web application framework) is at what I call the "high end of the web application framework food chain". I have also been getting into the Spring framework lately. Spring and Jaffa could not be more different: Jaffa is one large integrated framework that provides "out of the box" user management, administration functionality, and a fairly rich set of web app building widgets. When you use Jaffa it is a total commitment. Spring (using the dependency injection pattern in a light weight container, with a handy presentation framework called Web MVC) is very light weight - more for building up applications from scratch.
I have been also getting into Ruby on Rails lately - to me, using Spring is a little like RoR in the sense that it seems agile and is good for quickly and efficiently building small applications (yes, I know that other people use Spring for huge projects). Jaffa seems more tuned to maintaining dozens of linked corporate web applications in a controlled and modular fashion.