Expedia and Travelocity are making strides to ease the huge footprint of travel on the environment. Both sites have teamed up with organizations (TerraPass and Conservation Fund, respectively) that use travelers' donations to offset carbon emissions.

Before buying a plane ticket on Expedia, you can choose to purchase either $6 (short-haul), $17 (cross-country), or $30 (international) in credits, which will be used to fund a wind facility, a methane-harvesting dairy farm, and the Chicago Climate Exchange. Travelocity's Go Zero program aims to cancel out an entire vacation (flight, hotel, and car) by having the Conservation Fund plant trees using contributions ranging from $10 to $40.