Wood gas generators go back many years, and some US farmers and other folks with access to wood waste use them. Farm Show magazine had a recent article on a fellow who used his tubular pickup bed rack to cool the gas prior to burning. it works in spark ignition engines, but is really only useful for folks with great fab skills and lots of free wood.

Cool pic of an old tractor conversion:
Image:Wood gasifier on epa tractor.jpg - Wikipedia, the free encyclopedia

The Browns gas scam is famous, but do spend your money on a system and then hook up to an independent third party dyno under repeatable lab conditions and prove the naysayers wrong. I've decided to encourage everyone who believes in this stuff to put their money into it for the entertainment of the rest of us.