Hi, how can I easily create ground textures such as grass, snow or rock? Do you know links for free sources? thx

Check out http://www.3dtotal.com/textures_v7/ then go to there links and then textures. I've found some exellent textures using there links. Good luck

I used to go around with a digital camera taking pictures of surfaces, and then manipulate them in photoshop to make them tile and so forth.

Awhile back I wrote a little tutorial on doing what Fingers_ suggested. It can be found here. Basically, you're going to find it difficult to try to manually draw detailed terrain textures, unless you are one hell of an artist. But the real world is littered with fine textures of exactly what you want. If you lack a digital camera, many hi-res photos can be downloaded from: Mayang's Free Textures, ImageAfter, and other places... Just be respectful of others' bandwidth.

Good luck

You may like to give 3d Cafe a good look - many free ground textures are available.