RenderMan RIB Course - Course taught at the Polytechnic school - University of Sao Paulo in Brazil that introduces RenderMan RIB writing. Links to local copies in Adobe Acrobat format, by chapters.

RManNotes- Stephen May's great how-to for writing RenderMan shaders. A must-visit!

Take a look at some of the masters.:
Ken Musgrave's Home Page
Ken Perlin's Home Page
David Ebert's Home Page

Public Textures - When somebody offers photographic textures like these, you just can't say no. Nothing procedural, just really useful photos.

lumis.com- Hints, tips, tricks, tutorials, and a myriad of sample shaders, scenes, and scripts for Alias, Maya, and Softimage.

Graphica Obscura - They have a nice article on advanced texture mapping applications.