opmock
Mocking and unit testing tool for C and C++
...Or just download the docs, read them and compile.
The current version is 2.1. This is a full rewrite of the 0.9x series. The 0.9x is still maintained for bug fixes, but new functionalities will go only in the 2.0 branch.
Have a look to the blog, the wiki and the download page for more information.