GLSL
From Tech Artists Wiki
GLSL (OpenGL Shading Language), also known as GLslang, is a high level shading language based on the C programming language. It was created by the OpenGL ARB to give developers more direct control of the graphics pipeline without having to use assembly language or hardware-specific languages.
Syntactically, GLSL is quite different from HLSL and Cg. The dominance of learning material for HLSL, and the cross-platform abilities of Cg (which is similar to HLSL), means GLSL is much less popular amongst technical artists.
