MongoDB works great for that. You will need:
code:
import datastore_mongo_stub
os.environ['APPLICATION_ID'] = 'test'
datastore = datastore_mongo_stub.DatastoreMongoStub(
os.environ['APPLICATION_ID'], 'woot', '', require_indexes=False)
apiproxy_stub_map.apiproxy.RegisterStub('datastore_v3', datastore)
But if you're looking for truly faster development (like I was) the datastore is actually not the issue as much is the single threaded web server. I tried to replace it with spawning but that was a little too hard. You could also try to set up TyphoonAE which will mimic the appengine stack with open alternatives.
Be aware that if you do any of these, you might lose some of the exact behavior the current tools provide, meaning that if you deploy you could get results you didn't expect. In other words; make sure you know what you're doing :-)