Favourite Open Source Google App Engine apps (Java or Python)
To learn from good examples, what are the best open source Google App Engine applications out there?

I don't care if it is Java or Python based.

Please one app per answer. Feel free to add a link to the live app (if there is) and to the project page.
Favourite Open Source Google App Engine apps (Java or Python)
===

To learn from good examples, what are the best open source Google App Engine applications out there?

I don't care if it is Java or Python based.

Please one app per answer. Feel free to add a link to the live app (if there is) and to the project page.