[morganpyne]

+1 for SketchUp. It has many limitations but it so easy to get started with compared to the popular giants of 3d. Follow some of the tutorial videos.

It has a hugely active community writing custom plugins for just about everything you could imagine. My tipping point was finding all these plugins are written in Ruby :-) As a programmer I loved being able to hack together new extensions based on examining the source to other peoples work.

If you plan to do anything regarding animation, physics engines, or spline-based surface modeling etc then you'll need to look elsewhere of course. Blender is free and has a massive amount of features but a steep learning curve (and I personally dislike the interface).