Cg

• Cg: A System for Programming Graphics Hardware in a C-like Language - Whitepaper on the portable Cg shader language. Includes an abstract, background, design goals, and architecture. [PDF]
• Cg for Linux - Sample code and rendered examples of Cg using nVidia's Linux SDK. Includes standard examples such as bump mapping, refraction, grass, water and sprites.
• Cg In Two Pages - Mark Kilgard's quick guide on using the Cg shading language. Explains data types, vectors and comparisons with C and C++. [PDF]
• Cg Pixel Shaders in OpenGL - 3Ddrome's Cg Tutorial. Includes an introduction and sample code.
• Cg Toolkit 1.5 - nVidia's Cg SDK. Includes latest releases, specifications, documentation, tutorials and sample code.
• CodeSampler.com - OpenGL (1.2 - 2.0) game programming samples for Windows - CodSampler's list of OpenGL sample code, including many examples of Cg shaders.
• Shader Performance - Benchmarks comparing ARB, Cg and HLSL on nVidia GPUs. Includes source code, screen shots, and charts.
• Wikipedia: Cg - A brief history, description of Cg data types and sample code.