From Wes Duncan, the “High Tech Redneck,” a rocket stove made out of cinder blocks. I’ve built one of these too and can confirm that they work great. And you can’t beat the price. Time for some redneck cookin’!
Update: As several readers have pointed out, this design ain’t safe. Cinder blocks can explode and were not meant to be placed next to a heat source–that’s what fire bricks are for. See our post about our backyard rocket stove for a safe design that uses metal pipe.