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.