- z3c.traverser - Plug-in architecture for making custom traveral adapters. Joy! I believe lovely.tag, for example, uses this to get URLs like /tag/foo to work.
- z3c.zalchemy - Integration with SQLAlchemy, a lovely RDBMS and ORM tool. Note that Kapil has been working on somehing called Alchemist that also provides some SQLAlchemy integration for Zope 2/3/Plone.
- z3c.extfile - An external (large) file handling solution.
Therefore, it's probably wise to add svn.zope.org to the list of places to check before you start building something from scratch that may have been invented before. :)