Water, earth, wood, stone and steel. It is the combination of different materials that makes a gaming table great. We up in the frozen north have a gaming convention coming up in june and I'm trying to get a table finished which mixes all those surfaces.
Framing 4 mm plywood in L-shaped moulding and with some lightweight plates for shower framing (like giant sized foamboard) I hope to achieve a stabe table.
For surface material. The Croatian firm Tabletop World has the best stone terrain there is on the market, using real dressed stone for their projects.
German Thomarillion are also good on gaming terrain. Windows and doors saves work. As does some steampunky turbine and damp machine.
I also got some resin pieces from the american garage firm Rusty Rail producing terrain for narrow gauge O scale entusiasts. The details are suprisingly crisp and reasonably prized too.