From W3C Wiki

This is a list of concrete examples of Web applications and sites that might benefit from WebDescription; they may (but need not) already have a prose or machine-readable description of the interface exposed available. They don't have to be completely RESTful.

John Musser has a nice collection of Web APIs and examples, as does WSFinder.

Additionally, many protocols are built on top of HTTP, which should be considered for description as well;