PDF Alchemy is an interesting new service that I've helped to build. It's a PDF conversion service aimed at developers - that is, it's entirely API based, and not intended for end users. But what makes PDF Alchemy interesting to me, is the way it uses a number of Amazon's cloud services.
To start with, the PDF conversion servers themselves run on Amazon EC2. EC2 is an excellent match here, since it allows PDF Alchemy to easily, and rapidly, scale to handle almost any possible volume, but also to then scale back again during off-peak periods.
Also, PDF Alchemy makes great use of Amazon SQS queues to manage concurrency and distribution. Of course, SQS does not perfectly guarantee concurrency protection, etc. but applications like PDF Alchemy do not need such guarantees. Indeed, here we can trade off some of that guarantee for greater performance, scalability, and cost efficiency. So again, an excellent match.
As well as EC2, and SQS, PDF Alchemy makes significant use of both Amazon S3 and Amazon SimpleDB, both providing excellent scalability and cost-effectiveness in their respective areas, allowing the whole PDF Alchemy service to exist entirely in the Amazon cloud in a very nice way :)
It will be interesting to see how this service progresses.