Coding Algorithms for Procedural Textures?

Can anyone help me find how the procedural textures are implemented in the source code? Are the algorithms easy or complicated, so they could be used in own standalone Python-scripts? (Mainly I would be interested as “a case study” of the Cloud-texture.)

You could look at the example texture plugins included with blender.

well, the clouds texture is implemented Blender2.49source\source\blender\render\intern\source exture.c in the clouds() function. How easy or complicated that is depends on how well you read C I guess but it certainly is no case study.

A good book albeit pretty expensive is Texturing and Modeling: A Procedural Approach (The Morgan Kaufmann Series in Computer Graphics)

It is written by the inventors of many of the procedural algorithms now in wide use (like Perlin, Musgrave, etc)

A good web starting points might be:
http://freespace.virgin.net/hugo.elias/models/m_perlin.htm
or
http://www-evasion.imag.fr/~Fabrice.Neyret/textures/textures_bkm.html

An easier way is to simply use Mosaic and Aqsis.
That is what Renderman shaders are all about.
http://www.renderman.org/RMR/