Java Persistence Frameworks.
The Blogging Roller has posted a nice chart comparing some of the most popular persistence frameworks. The chart makes Castor look pretty good. I wish one of these frameworks had the same popularity as Struts, then I could just learn it and use it. As it is, it doesn't seem that one stands out above the other. However, I bet one could become the "market leader" by simply putting together a good set of documentation and how-to's. That seems to be the biggest problem with open-source solutions - lack of documentation that works. However, at the same time, a mailing list with lots of subscribers can solve most documentation issues.