Procedural texture generation

A lot of different simple algorithms would be layered on top of each other there.

I'd suggested downloading MapZone and trying to create some textures with it. After that you'll have a better understanding of the layering process involved.

the main technique however is likely perlin noise.

Using various levels of noise, one can map colors to greyscale values, creating such patterns.

The the other effects, like the dripping rust, is likely created by taking mulitple samples of the final "rust" area and dragging it down and fading out. the bump map (which in my opinion doesn't look good in this sample) can also be created using the same noise technique. Then the result is lit (in this case probaly using emboss technique, but it could be done using normal mapping for better realism...in procedural textures you need to keep in mind the difference betwen color maps and normal maps/lighting effects--depending on what the ultimate purpose of the texture is).

First of all, thanks for the replies.

Well, my initial guess was that they used Perlin noise. But I'm still wondering what underlying pattern/technique was used, as it is not a "complete noise".