Not sure if anyone has posted this yet, but Google has a free 3D sketching software: http://sketchup.google.com/
It's not designed for cake, but if your willing to get acquainted with the software it seems to work really well for the basic needs of a cake sketch. Not sure how it compares to cake sketching software. In this you'll have to make everything from scratch. I have CAD experience so it seems easy to me, but give it a try it's pretty basic!!
Select "Google SketchUp 7" if you choose to download it for free.