We just released our API on apiary (http://docs.commando.apiary.io), but after finding ReadMe.io on ProductHunt, we signed up for a paid plan that very day, and are in the process of migrating over.

Our focus for our API documentation is beautifully designed and easy to read. All the extra features of Apiary and Mashup such as proxying, rate limiting, billing, and mocking requests are not really a priority for us. ReadMe.io is a perfect fit.