WhatYou is a simple Twitter clone with a twist, in that you can post several status messages at a time, and set an automatic timeout period on each one.
WhatYou was developed for the purpose of learning and understanding the Google App Engine service.
Licence:
GPL v2
Production Link:
http://what-you.appspot.com/
Source Code Link:
http://code.google.com/p/whatyou/ 