CMock is a module/object mocking framework for C projects. It generates dummy modules that conform to the interface specified in a header file. This allows APIs to be proven out and exercised before committing to an underlying implementation.
CMock project hosting has moved to GitHub under the ThrowTheSwitch Organization: https://github.com/ThrowTheSwitch/CMock
http://throwtheswitch.org contains extensive content and support for CMock and other awesome tools.