Sketchup is a free 3d tool from none other than the great Google.  I decided to try and learn to work with it, since it is really quite nice and I see plans drawn up in it all the time on the internet.  The process was painful learning when my only other experience with 3d was autocad about 15 years ago.

Edit: It looks like Google sold sketchup to a company named Trimble.  I wonder what this means for the future of the product.

This is a preliminary design of my black pipe bathroom shelf.