After talking with the folks at Stanadyne (makers of the fuel delivery and filtration systems on the Hummer) I began using biodiesel fuel in my truck. It burns cleaner than pure petroleum diesel, cuts down on pollutants, as well as improving the effective cetane rating and lubricity of the fuel.
In the past I have purchased biodiesel at Solar Market
More recently, a local fueling station has started to carry b20 at the pump, and some local oil delivery companies provide a "doorstep" delivery service as well.
For more info regarding biodiesel use, check out BIODIESEL.ORG
Also, check out this article I wrote for OFF-ROAD.COM,
"Biodiesel: Green Streak for the Hummer H1"