SkyTruth became famous during the BP oil disaster. They used satellite radar images to provide better estimates of the spill rate than what was coming out of BP and basically forced the government's hand to study it in more depth. This had an enormous impact on the scale of estimated damages and it was important to do the work while the spill was underway.
They are still active using imaging in defense of the environment and are very involved in fracking surveillance. A fine organization to do some volunteer work for - you can do it from your desk - and one worthy of donations. And their blog is often fascinating.
from their "about" section:
SkyTruth is a nonprofit organization using remote sensing and digital mapping to create stunning images that expose the landscape disruption and habitat degradation caused by mining, oil and gas drilling, deforestation, fishing and other human activities.
Our vision is a world where all people can see and understand the environmental consequences of human activity everywhere on Earth, and are motivated to take action to protect it.
Our mission is to motivate and empower new constituencies for environmental protection through illuminating the issues that impact our planet.
We use scientifically credible satellite images and other visual technologies to create compelling pictures that vividly illustrate environmental impacts, and provide these pictures and supporting data to environmental advocates, policy-makers, the media, and the public.