We are very nontraditional in the way we do this. We typically donate to a charity that plants trees like The Nature Conservancy's Plant A Billion Trees program or Heifer International's Trees Are A Treasure program. The Plant A Billion Trees campaign is planting trees in Brazil's Atlantic Forest in order to restore/preserve 30 million acres of rainforest in the next 2 years. How awesome is that?!? And The Trees Are A Treasure program often plants fruit trees not only to preserve topsoil but also give nutritious food to families providing for themselves. (The vegetarian in me loves this part!)
If you'd like to go a more traditional route to offset greenhouse gases produced through travel, you may prefer The Conservation Fund. It even provides a "Go Zero" calculator. (Very eye-opening!) Another one that I've heard great things about (but never used) is Carbon Fund. I love that it has a whole section devoted to reducing our impacts listed before the offset section.
Which others am I missing?
Peace,
Stacey