In November 2004, I gushed a little bit about Amazon's Simple Queue Service (SQS), a web hosted, reliable, persistent, messaging queue. Back then it was free, but beta. Now the SQS has gone production, meaning the feature set is stable, you can get an unlimited number of queues, and it costs money. But SQS is cheap on the scale of Amazon's S3.
Speaking of which, SQS and S3 start to make a nice foundation for building Web applications which I'm sure is Amazon's intent. You provide the compute engines and user interface, and they host the hard parts of the web scale infrastructure.
I've said before that I didn't think S3 was a game changer, but Amazon's web infrastructure strategy might be.