POV-Ray is a freeware raytracing program available for almost any computer (I haven't found a version for the TI-86, though).  For more information, see the POV-Ray Homepage.

• Wavelength-dependant Rayleigh scattering
• Infinite-light hack -- needed to make wavelength-dependant scattering look good
• A few notes on how to use the program.
• Two or three demo scenes

Include files I have created:

Real Units include file:
This file allows you to use real-world measurement units in your scene files, and allows you to use objects created by other people using the include file without needing to scale those objects.
Units.zip

Vegetate object placement routine:
This include file will place objects at random on the surface of any object.
Vegetate.zip (10K)

Example output: