The MoinMoin project is developing a popular wiki engine in Python.
We are currently making big changes to how MoinMoin works (for moin2) and have lots stuff to do, so contributions really matter to us.
Wikis are getting more and more important for the communication infrastructure of OSS groups all over the world. Especially groups like Apache, Ubuntu, Python, Debian, Xen, KernelNewbies, linuxwiki.org (de), etc. are using MoinMoin to keep the contact going with their users and developers and for documentation.