Long story short: Google App Engine is Google’s public application hosting service. You can run your own web application, databases, etc. on Google servers. It’s free for small applications.
Of course you could run your own server (either rent one or run one at home), but this means you have the maintenance problems, security issues and performance problems to deal with.
Unfortunately I have not been able to attend JFALL this year, there was a session which was about Google App Engine.
I have some ideas about using Google App engine. Currently I am thinking about using it to store data for my Android applications.
It’s definitely something I’m going to look into very soon.
This guy got a small Wicket application running on App Engine, cool eh!