DataRush is a light-weight (less than 3 MB on disk) but extremely powerful parallel processing engine framework. It's 100% Java and runs on Java 5 SE. It handles all the parallel programming for you including horizontal, vertical and pipeline parallelism
DevX.com has a nice series of articles entitled the 'AJAX Framework Roundup' where they walk through all the frameworks and discuss pros and cons. Nice Prototype tutorial included as well
