Cg

From Tech Artists Wiki

Cg is a shading language created and maintained by NVIDIA. It is syntactically very similiar to HLSL, but is cross-platform (DirectX and OpenGL), which makes it very attractive as a shader language- it can use HLSL's vast documentation but has the benefit of cross-platform compatibility, like GLSL. However, because of the widespread documentation and support for the fx framework and HLSL, Cg, and its CgFX framework, is much less common than fx and HLSL.