http://sketchup.google.com/
Anyone interested in a sketching prog. Have a look at this.
Tis very good for a free download - fast to learn and use, and capable of some reasonable modelling given that it's free and will run on the average family PC. I'm using it for parts of my PhD and am also hoping to do a paper on it shortly: "Archaeological 3D visualisation for the terrified: Using Google Sketchup to model the Neolithic timber longhouse at Claish".
It's also a great toy to while away an afternoon with ;D