Blender is an amazingly powerful 3d suite (equivalent in terms of features to professional products costing thousands of dollars). It's stable, quite small (30-40MB installed), runs well on modest hardware, and can easily be installed in a few minutes.
I use this program occasionally, but I find its interface remarkably obtuse, even by the rather poor standards of 3d software. Learning to use this program properly is not for the faint-hearted.
But even for the occasional user, such as myself, Blender offers amazing capabilities, including full scriptability (via Python), support for a wide range of 3d file formats, and very high end rendering functionality including volumetric lighting and subsurface scattering.
On the plus side, there is a pretty large and healthy online community (but don't complain to them about the user interface, they love it).