Well, you might now ask, "Well, how do I use these in Blender?" Well, here is your answer!
1. Set the Shading Mode to 'GLSL' under 'Display' on the properties panel (n-key)
2. Add a material to your object
3. On the shading panel of the material tab set the 'Shadeless' option to true. This makes it so that lights on the scene don't affect your object.
4. On the texture tab select 'Image or Movie' under type and then browse to your matcap file.
5. On the Mapping panel select 'Normal' under coordinates. This makes it appear as if lighting is coming from the viewer. So, basically any which way you look at the object, it will appear lit up.
6. Enjoy the good viewing experience!