Updated Sept 16 2008: Note that this is not a complete list of Java workflow applications, just the ones that I have found and interest me.

One for the biggest of open source "enterprise solutions" for Java is ADempiere Business Suite. I haven't had time to evaluate it and probably won't have time in the near future. The nice feature is that it has not only a fairly flexible workflow engine but also a solid core of business modules for accounting and basic "Enterprise Resouce Planning" functions.

ADempiere had not been active for a while, but it's livened up with a series of new releases. Large business application suites tend to run on a longer release cycle than other applications.

Another gigantic ERP application ( >100MB ) is Apache OFBiz, with a stronger web server/developer orientation than ADempiere. It is possilbe to find versioned source code downloads maintained by indivduals but developers are expected to work directly from the Subverion repository. That makes it harder for ordinary mortals; to use ste up an OFBiz site. The impression is that the project is not very active.

On the the hand, OFBiz also has a better defined workflow/rule engines, though they seem to be getting depreciated for some reason, at least in the documentation.

A relaitively small and powerful contender in the Java workflow arena is Spring Web Flow, integrated into the Spring MVC framework. For a tutorial about Spring Web Flow.