It would be good if we could collect together a library of GLSL code that we would like our Shader to be able to process. This should be helpful both in the design stage of the Shader and later on for testing.
A very useful starting point is the ShaderGen program from 3dLabs. The program only supports OpenGL 1.2, but is still very informative.
A second source of small shader programs is the GLSL examples supplied with the mesa source code.