DoubleCAD XT is the one of the finest, quickest, and most complete alternatives to Autocad that I've ever tried, but the biggest difference with other options available in the market is that their free version is uncrippled, unexpiring, fully-featured, and best of all, free for commercial use. It's really a unique program.
Info and download here:
http://doublecad.com/Products/DoubleCADX...fault.aspx
You'll need to register in order to get your serial and activation code.

regards,

Alvaro