Rayshade is an extensible system for creating ray-traced images. It is written in C, yacc, and lex, and runs on many different platforms. More information on rayshade is available.
The most recent source distribution of rayshade can be found here.
Online rayshade resources include:
The Rayshade FTP Archives
If you have trouble using the above URLs, the archives can be accessed via anonymous ftp as graphics.stanford.edu:pub/rayshade.
The Rayshade User's Guide
The Rayshade quick-reference sheet
Example rayshade images
The Rayshade Gallery
Rayshade Image Exhibit
The rayshade-users mailing list
Notes on Rayshade
Rayshade Objects Library, courtesy of the University of New South Wales, Sydney, Australia.
The Utah Raster Toolkit