CuTest lets you write unit tests for your C code. You should use it because: (1) It has the cutest name, (2) It looks and feels like JUnit, (3) It is cross-platform, (4) It ships in a single .c and .h file for ease of deployment.
The previous generation graphical download manager
yoDownet is an open-source && free graphical download manager project that tries to be simple and minimal, also for sake of humor it's described as "The Previous Generation Graphical Download Manager".
Colour Management System with integrated printer driver. Uses ICM profiles as specified by the International Color Consortium.
Software for the 3D modelling of environments at relativistic limits (OpenGL based).