Pacific Gas and Electric is now offering to help its customers become carbon neutral. Through a service called "Climate Smart," you can sign up to pay a "carbon tax" on the energy you use. While it doesn't cover your gasoline consumption or any energy you use outside of your house, it makes it easy to take care of one small aspect of your overall carbon footprint.
It's also inexpensive. I haven't compared it to other services, but the amount is so insignificant that it's not worth the time to compare. Using this calculator, I determined that I will only be adding about $1.40/month to my bill. Granted, I won't be paying anything for electricity as my solar already covers that. The best part is that the tax is calculated for you and simply added to your bill. No extra work. I love that.
If you live in California and you are a customer of PG&E, check out this program and see if you can't cough up a couple extra dollars a month to cover your energy consumption. You might even make it a game to see how low you can get your carbon tax in any given month (like seeing how high you can coax the mileage on your Prius).