Google App Engine enables you to build and host web apps on the same systems that power Google applications. App Engine offers development and deployment; simple administration, with no need to worry about hardware, patches or backups; and effortless scalability.
The following applications are doing pretty well and have recorded impressive statistics. They were all build on the Google Apps engine.
BuddyPoke! Express yourself! Hug, kiss, tickle, or punch your friends with your own personalized 3D avatar.
Panoramio- Photo-sharing community. Discover the world through photos.
Mentez is one of the leaders in social games and applications in Latin America with 22 million monthly active users. Mentez handles over 600 transactions per minute.
Socialwok - Increase team and company productivity! Share securely ideas, email, files and Google Apps in your enterprise microblogging network. 20K plus organizations use it daily.
cricwaves.com Provide a flash based Cricket Simulator of the ongoing live matches! Its displays the animated shots played by the players .. LIVE!!!.. A whole new cricket experience online… with Live scores, scorecards of previous matches, series focus, team history, Player stats, ICC Rankings and Add Live Score widgets for your Website / Blog. At peak times they have tested 500 requests/sec,
Checkin Mania: Google Map Mashup of Foursquare, Gowalla and other location based socail media services.
MisoTrendy | Find out where people are checked-in now.