Mostly it's word of mouth, SEO and the occasional writeup on Amazon's Web Services blog. And this:

http://www.s3stat.com/web-stats/cheap-bastard-plan.ashx

It's a strange niche to be in because it's something people are looking for but there was basically nobody doing it when I started out. It goes squarely against the two most fundamental pieces of business advice you can get: "Never build a business based on a missing feature of a popular software product", and "If there's no competition in the space, be warned: there's probably no market there".

Evidently, there are actually a few good left ideas out there that nobody ever thought of doing before.