I've been trying to create an eye texture with blenders texture nodes, without the need to import an image texture.
The iris texture is created with a Marble node, duplicated and rotated and added to the original, and so on.
Have fun improving it, and I hope you share your results.
Wow, just up and 9 downloads already, just wanted to add that the material is just applied to a default sphere, the highlights and shadows are all done with texture settings, the material is shadeless.