The only programming language supported at the moment is Python. Google offers 500 MB of storage and enough bandwidth for 5 million pageviews per month for free, but in the future you'll be able to purchase more resources. Here are the expected prices, according to the App Engine Blog:
* $0.10 - $0.12 per CPU core-hour
* $0.15 - $0.18 per GB-month of storage
* $0.11 - $0.13 per GB outgoing bandwidth
* $0.09 - $0.11 per GB incoming bandwidth
For comparison, Amazon SimpleDB costs $0.14 per machine hour, $1.50 per GB-month of storage (8-10 times more than Google App Engine), $0.1 per GB incoming data transfer and $0.1-$0.17 per GB outgoing data transfer. Except for the storage cost, the pricing is somewhat similar to Amazon's offering, even if the services are quite different.
If you need ideas for an application, browse the gallery - you'll find a lot of simple apps for URL shortening, reading lists, unit conversion, time management or just for fun. You should also check Niall Kennedy's article about App Engine before reading the documentation.
"Google App Engine is a new and interesting solution for Python developers interested in adding features, not servers. Google spends hundreds of millions of dollars developing its custom infrastructure with 12-volt power supplies tapped into a hydro-electric dam next door and fat fiber pipes owned by local governments carrying requests and responses to their proper home. Google's physical infrastructure is vast array of highly optimized web machines, and we'll now be able to see how such infrastructure performs across more generic applications on App Engine," writes Niall.
{ Thank you, Tim. }