Free!
Open source 3D modeling software, CAD type. Mainly focused on architecture and mechanical engineering.
Free with limitations
Sketchup (formerly Google Sketchup) is 3D modeling software easy to use with a huge user community and 3D model archives available.
Free!
Nice 3D program even for beginners, it allows you to create quickly 3D models, even for printing. Possibility to render with Lego or Minercraft look :)