There are web services mocking tools like latest SoapUI 5.0, focused on improving the REST testing capabilities, and UltraESB mocking a typical ESB in SOA environment.
Now there is WireMock, an extra light weight tool by creating an actual standalone HTTP server response stubbing, or embeded within an application, even as mocked stub in unit test.
+Dean Budd Deano initiated with a file system based mock services under his Open Tomato Sauce licence. It's same idea, but he couldn't finish it as a generic app.