Side note: It's funny that when these threads come up, a dozen people will all suggest different programs and they all turn out to be made by Autodesk. I'm pretty sure Autodesk makes one million programs, and of those million, ANY seem to be a good choice from what I've seen.
(FYI, 123D Design is an Autodesk product. So that puts the count at 2 so far for this thread. Stay tuned.)
-Rog