Theres a few free CAD apps out there, none of the free ones are going to pop out a 3d house though. Since I use the stuff that costs 3 grand plus per license I haven't spent much time looking at the free or nearly free stuff.
One of the nearest clones of AutoCAD is called ProgeCAD. Its free, unlesss you want 3D capabilities. They have an Arch add-on for about $300 if I remember correctly.
But I doubt you want a full-on drafting program. So let me point you to a free online program that Autodesk has out there. Its pretty much just a toy compared to their other sttuff, but it might be good for what you want to do with it. Its strictly 2d though. Its called Project Draw, and lives on their labs website.
http://labs.autodesk.com/technologies/draw/
It is entirely server based (cloud computing is the buzzword) so no install necessary. Just open the website and draw.
-Steve G.