OpenGL related Books
The 'red book' (start here when in doubt) http://www.glprogramming.com/red/
OpenGL Programming Guide for Mac OS X http://developer.apple.com/documentation/GraphicsImaging/Conceptual/Open...
Programming with OpenGL: Advanced Rendering http://www.opengl.org/resources/code/samples/advanced/
GPU Gems 1 http://developer.nvidia.com/object/gpu_gems_home.html
GPU Gems 2 http://developer.nvidia.com/object/gpu_gems_2_home.html
GPU Gems 3 http://http.developer.nvidia.com/GPUGems3/gpugems3_pref01.html
OpenGL related Tutorials
Transforms & Drawing http://www.codecolony.de/
Step by Step, from a Triangle to more complex scenes http://nehe.gamedev.net/
Lots of Extension-related Examples http://www.codesampler.com/oglsrc.htm
Misc. advanced Tutorials http://www.opengl.org/sdk/docs/tutorials/Lighthouse3D/
GLSL related Tutorials
Compilation, Linking & State http://www.opengl.org/sdk/docs/tutorials/ClockworkCoders/
Material & Lighting http://www.typhoonlabs.com/
Articles, Demos & Research Papers
Old Website, Forum is a goldmine http://www.gamedev.net/
Misc. advanced Demos /w Source http://www.humus.name
Voxel & Raytracing http://www.codermind.com/articles/Technical-articles.html