Issue Tracker webapp designed by Ka Ping Yee.
http://roundup.sourceforge.net/ includes a Zope interface.
seems active (as of May'2008)
Possible piece of a Collaboration Ware suite, in combination with a Wiki.
On aspect widely lauded is the use of the Nosy List concept to combine the benefits of and EMail list and generic ad-hoc threading.
Has its own Hyper Db Data Store which can be used on its own.