In Dana's Gallery you can find high quality images to a couple of different topics ranging from exotic plants over landscapes to animals. These pictures are very useful as reference for modelling 3D plants (for example with L-systems) but serve also quite well as distant background images for 3D games (i.e. sky domes etc).
This is a great site to learn about the physics involved in the simulation of nature. You will find a wealth of information here, especially his fluid solvers are worth to take a look at!
Most people probably know Robert Platt from his famous 3D Boids project, but that is just one his many interesting projects. He has also designed a set of OpenGL GUI components which offer great potential - everything in the high quality he is known for!
Dog Screensaver is a little screen saver that simulates "dogs" motions with a simple particle system. Particles have a head and a tail, they can be in seeking or frightened state. If a particle is not smelled, it seeks for the nearest butt around. If it is smelled, it goes straight with increasing speed...
Craig Reynolds invented the first model of coordinated animal motion such as bird flocks and fish schools in 1986. Today you can admire the results of his work in award winning films like "Stanley and Stella in: Breaking the Ice" (SIGGRAPH 1987) or "Batman Returns" (1992). On his site he explains how his model works and you can find all relevant links related to this topic.
WWW.OPENGL.ORG is the biggest site about opengl. Here you can find everything from opengl books/papers over opengl demos to opengl source codes. Very informative is also their news section, where you can find the latest news from the opengl community (new products, demos,...). This site is highly recommended.