Peter's been keeping us up to date on California diesel prices (supposedly), and they're high.  I've found a solution for his Jetta.  It's a conversion kit for a diesel-powered car that allows it to run on vegetable oil.  I had seen this concept before, but it was experimental.  These folks sell the conversion kit.

Oddly enough, vegetable oil prices are much lower than diesel prices, especially if you parter with a local fast-food restaurant that would have to pay to get rid of it anyway.

Looks like GreaseCar.com sells the kits for around $800, which would more than pay for itself in a short period, especially if you're getting the oil for free.  Evidently it burns more cleanly as well, a boon for any environmentalists out there.

I may have to find me an old diesel-powered car and do this as a project!