Someone asked for a tutorial about sphere maps, and at the same time I thought it would be worthwhile debunking some of the misinformation that's floating around about what these types of graphics actually are and how they are used.
More info (if you want the maths): en.wikipedia.org/wiki/Sphere_m…
Image used for example sphere map from wallpapersup.net/
Model used is my own edit of the default Meiko SakineErrata
Made some minor corrections (12/14/2012)
Note also that if you're model a PMX model, there are more options with what you can do with a sphere map.