Blender. It doesn't sound like a real significant piece of software. Maybe it sounds like a little sound mixer, or maybe a small photo editor. But Blender is none of that. It's a full-fledged, surprisingly powerful, and inspiringly agile 3D Modeling program, complete with a thorough manual, wiki, support forum, and tutorial collection to get you well on you way towards professional modeling.
From the Blender website,
Blender is the open source software for 3D modeling, animation, rendering, post-production, interactive creation and playback.
And Blender more than lives up to it.
Just look at some of the screenshots of renderings made with Blender I snatched from the gallery:
Blender is released under the GNU Public License, and the source code is easily available on the main site. The Windows executable is small (only 6.5MB!), so it's no big deal to download Blender to give it a try. The entire GUI is OpenGL, and the interface is spectacularly intuitive; don't let the initially daunting appearance fool you. It is one of the easiest programs to use once you get a hang of the shortcuts and redundancies, in spite of the plethora of features in such a small program. Blender seems to be one of those golden OSS programs that has not become bloated or inefficient or buggy, and is the epitomy of what Open-Source should give the world.
I don't want to try to describe the beauty (both art-wise and programming-wise) and power of Blender. Download it yourself, and start experimenting. It's so natural, that even a complete non-artist like myself can create some pretty sweet renderings. Have fun!