Ask HN: Which idea to use to illustrate building a startup on EC2?

I am writing an extended online guide on how to build a startup company from scratch using tools from Amazon and Google, covering EC2, S3, Adwords, and the like. Any ideas on which startup idea I should use? The idea does not have to be original, in fact it is better for design comparison if there is more than one company pursuing this idea. Also, it should involve some online data storage (so that I can show how S3 is used) and online processing (for EC2). There's the classic online photo/video sharing app. Anything else?