Fundamentals

Ldraw

Ldraw.org (Direct link to a page where you can download everything listed on this page at once)

MLCad

http://www.lm-software.com/mlcad/

L3p

http://www.hassings.dk/l3/l3p.html

POVray

http://www.povray.org/

(You'll find more tools and links at Ldraw.org)