I'm not up to speed on how compatible and recent it is, but AppScale bills itself as "Open Source Platform for Google App Engine Apps": http://code.google.com/p/appscale/
in addition to AppScale also look at the TyphoonAE project... these are open versions of the App Engine backend that claim to be fully-compatible to the SDK/client-side.
also to fight vendor lock-in, Python developers can create Django apps then run them on App Engine using django-nonrel or on any traditional hosted stack supporting Django; or move between then w/minor (settings.py) changes; don't forget data migration as well.
