I have used 3D Crafter (formerly 3d Canvas) from
www.amabilis.com for longer than I will admit to!!
I started using it when I was producing models, etc for Microsoft Train Simulator as it has a built-in exporter that created the correct files for the sim. As I knew the software, I've never looked at any other package basically because I've been used to the package for so long, it feels comfortable!
3D Crafter does have its foibles, though. It's Boolean logic is not that good and the package slows down dramatically as the poly count gets very high....but I'm happy to live with things.
3DC has a free version that gives you a range of basic tools...and then there's a Pro version that costs $40, which you'll need to export stl format files for 3D printing.