I just stumbled upon this yesterday. It is truly the Ultimate World Texture.

Taken by NASA(US Space Program for those who dont know it) by thousands of satellites. Not sure how long it took to string together but its amazing. On the highest version (not sure if its on the website) each pixel of the image is equvilent to 1 kilometer. So if you are lucky you can see your home town.

You can download the land surface, oceans, sea ice, and cloud images and layer them in a shader to create an extremely realistic Earth.

Check it out here: Earth's Observatory