Google App Engine gives you access to the same building blocks that Google uses for its own applications, making it easier to build an application that runs reliably, even under heavy load and with large amounts of data.
It's a pretty interesting platform... user authentication, load balancing, database all for "free". It's not tempting enough to learn Python yet, but they say there will be more supported languages in the future. It's certainly compelling enough to finally learn PHP. This would be a home run if it supported Java (unlikely).
