SketchUp (http://www.sketchup.com/) – Free for personal and educational use. Great program for modeling objects, especially architecture. Works on Mac and PC (Windows 7 & 8).
AutoDesk has a lot of cool creativity resources. Much of it is aimed at professional markets, but a significant amount is free for personal/educational use. Try Formit360 for an online and Android/iOs 3D model maker.
