I'm using this stuf on a boat project, thought I'd share because it lets you make all kinds of stuff that would normally require a mold.
33" beam, weighs 19 oz, holds my 200lb ass:
http://img94.imageshack.us/img94/5143/planku.jpg
You basically just make your shapes out of foam, I use the pink owens corning stuff you find at home depot.
You make your shape, than layer fiberglass cloth and West Systems epoxy over it. Instead of vaccume bagging, I just wrap the stuff in a plastic that the epoxy wont bond to. For flat stuff, I'll compress it with a couple planks of wood and some clamps.
The above beam is four slabs of 1" thick foam with fiberglass between each layer, then whole thing is wrapped in two more layers. the 5th peice on top is a spacer for my specific aplication.
Use your imagination and you can make almost any shape you want out of this stuff.