At A Year of Frugal Gaming Blog, the author has an illustrated article on his efforts to create a silicon mold to allow him to replicate various architectural parts for terrain making.
Terragenesis has a nice article on casting bunkers in plaster from chocolate boxes.
The Toy Soldiers Forever blog has a video on proper techniques for casting your own toy soldiers from pewter. He also suggests some cheap sources of the metal.
This tutorial is on casting prop coins from polymer, but on a more miniature wargaming theme, I see this as a great way to cast casualty and morale markers.