I visit Carlos E. Perez's Manageability blog about once a week because he occasionally posts useful list of open source Java projects along with terse yet revealing comments.  I thought it might be useful to list the lists.

• Portal Servers (most recent)
• Workflow Engines
• Rule-based Engines
• Regular Expression Engines (benchmark)
• Two Fast XML Parsers

Carlos is a bit heavy on the Java cheerleading, but these gems are worth the visit.