If you have to buy gasoline, the Sierra Club has recommendations
Bottom of the Barrel
Middle of the Barrel
* Royal Dutch Shell
* Valero Energy Corporation
Top of the Barrel
If you happen to think supporting Chavez will get you thrown out of the Lodge, don't buy Citgo. Me personally, I've been going to Hess because of the price and proximity. I drive a hybrid and put in about 9,000 miles a year. Half the time I ride my bicycle. Hopefully I'll keep riding my bicycle at the same rate and perhaps more this year. And when I do buy gas I'll look for BP.
Greenpeace was attacked by special interest groups regarding its finances. IRS conducted an audit and it turns out everything was ok. The special interest groups turned out to be entirely funded by Exxon Mobil. Alternate views on greenhouse gases and global warming were funded by Exxon Mobil, leading public doubt on the environmental concern poised by conservationists.
Exxon Secrets has a handy flash site mapping various ExxonMobil shenanigans bent on keeping their coffers filled.
Correction: Since March 2005, I've only driven it 12,000 miles. I'm averaging about 3,500 miles a year.