Hirst Arts, run by Bruce Hirst, and his wife produce high quality silicon molds which can be used to produce plaster and resin pieces, which when glued together can be used to make 3d gaming terrain. It's a very addicitve hobby. The molds really are of very high quality, and are reasonably priced, with discounts offered for bulk purchaes. Bruce has a wide selection of molds available. Casting the molds is a relatively easy process, if you follow Bruce's step-by-step guides (available in video and print format on the website). In fact the hardest part, is probably selecting which molds you want to buy next!
Below you can see the images of the casts that can be produced by the Hirst Arts molds that I currently own.