Google Sketchup is the crazzy prog of these days (and free for almost all functions)
It has an amazing "Photo catch" who permited to project any photo on "a modeling" over this same photo! (It's the not the image linked folow)
A crazzy "Style" function (draw as with a brush or a marker)(It's the image linked folow)
And its egonomy is surely the best (with Moi) at this hour :)
learning curve = 15 minutes !
A super task for quicky architectures urbanism and quick shematic design object with "measures" writed !
Kids of these days are very lucky! Absolutly amazing!

http://sketchup.google.com/gsu6vtvideos.html see video 2 3 4 5
(other video about "Layout" is only for the Pro version)
Model + a "Style" !) Crazzy! As soon as the format "OBJ" Object will be directly implemented that will be a "Bomb" :D