As I mentioned in my 3d basics for Manga Studio post, Blender is a robust, full-featured 3d package that's absolutely free. And if you only want to use 3d to create the occasional prop or background, it doesn't make a lot of sense to invest in expensive tools like Autodesk's Max, Maya or Softimage.
For many years, Blender users suffered with a really terrible interface. That UI has improved dramatically in recent iterations and is fairly consistent with other 3d packages, except that it relies on a huge number of commands that are only accessible via keyboard shortcut.
As much as I love this stuff, I haven't been able to muster the energy or brain cells to learn Blender well enough to write about it intelligently. Fortunately, there are a lot of video tutorials out there and the supply seems to be growing daily.
I just came across this great video walkthrough by Paul Cageggi, who produces the web comic Pandeia using Blender for 3d content. I highly recommend investing the 30 minutes to view it.