I've been working on some simple forum software and it's almost ready to be released.
You can find a demo here: http://flowerace346663.appspot.com/forum/28002
The idea is that it attempts to run within the daily limits for app engine free accounts, so anybody can set one up without having to hire a server etc. And as long as your forum stays relatively quiet you won't have to pay a penny.
You can find quota details here: https://developers.google.com/appengine/docs/quotas
But it makes more sense to think about it for this software in terms of pageviews and posts. And I don't have those figures yet.
This will be free software for non-commercial usage with a parallel track for commercial usage or paid support and feature requests.
Programming side is mostly done, just the design left to go.