Reqr.es
reqr.es
reqr.es
I'd like to point out though, as I always do when things like this, I'm in the maintainer of a similar project which isn't hosted but is an open source node project: http://github.com/basicallydan/interfake - if you wanna do a similar thing but make responses more explicit. About the same amount of work up front, but you don't specify responses in requests.
Anyway, nice one Ben!
Would be nice to be able to customize keys, choose a format for a list of a resource, etc. so that if you use this in development you don't need to make changes for your "real" API if you have different standards.