Google Sketchup (MY PICK!)
For Beginners to advanced users – http://sketchup.google.com/download/
In my opinion, this is for the casual to serious designer that intends to print their designs. It strips out a lot of the confusing theory behind advanced modeling, but is deep enough to be capable of surprisingly complex projects. There is a “PRO” paid upgrade that I don’t think you need at all until you really feel pressed for more features. Even though I have SolidWorks ($$$$), I prefer Sketchup for its accessibility.