Chicago resident Jenna Rose made it a New Year's resolution one year to pay for the carbon she produces in air travel. Through terrapass.com, she calculates how much carbon dioxide she is personally responsible for as a result of her flights, and how much it would cost to offset those emissions through renewable energy. So far she's paid a total of $37 for two domestic trips and one international flight, which will go to buy carbon credits that fund clean energy projects like wind energy farms.
In addition to TerraPass, you can try carbonfund.org and gocarbonzero.org.
You can also buy "green tags" directly from companies with renewable energy projects as a way of supporting those projects. Visit green-e.org for a list.