LDraw is a software program, originally for
DOS, for creating models out of
LEGOs. It was created by
James Jessiman (who passed away
July 25, 1997) and is currently maintained, in some fashion, by the folks over at
http://www.ldraw.org.
Many people have used LDraw-created .dat files with a program called L3P (by Lars C. Hassing) which translates them into pov-ray-renderable 3-D scenes, from the command line. Also useful is the older L2P.
Various LDraw-compatible but more advanced CAD programs exist, including the ldraw.org-approved and 100% ldraw-compatible MLCad by Michael Lachman, and LeoCAD, which does not use the standard LDraw parts files but can export a LDraw .dat if necessary.
I much prefer the ease of use of LeoCAD, and as it is built for Linux as well as Windows few PC users would be left out of the Lego-luvvin'. Find it at http://www.leocad.org. MLCad comes from http://mlcad.ldraw.org, if you're interested.
You can see some examples I've created with LeoCAD and rendered, at Brickshelf, at this address:
http://www.brickshelf.com/cgi-bin/gallery.cgi?f=776
Mac users: Feeling left out? Try BrickDraw3D, a LDraw-compatible program for Mac OS 8 and up. I can't say much more about it, having not tried it, but if you want to give it a shot find it at http://olson.pair.com/brickdraw3d/.
And have some fun! Remember, LEGOs aren't just for kids anymore!