Be Careful Where You Go Green: Znout vs. Greenseng Search
Znout (Zero Negative Output) has opted for a low-power black background, but unlike Blackle you can change the color if it's really bugging you. Advertising is a bit bothersome, with a large AdSense block atop the page and two sponsored links below, both before your search results.
You'll also notice the Znout favicon replaced by the little blue G, which leads me to believe that Znout has been hastily knocked together to grab some advertising dollars.
Greenseng, on the other hand, displays only the two sponsored links above your results. They've also left all Google's links - images, maps, shopping, news, etc. - intact.
Both Greenseng and Znout display the CO2 Stats "Green" badge, but only Greenseng's actually opened a certificate page. I'm much more convinced that the Greenseng team is doing what they do for the right reasons.
Since there's such a buzz about being environmental, make sure you take claims of being green with a grain of salt. Do a little investigating, and be sure you're supporting the right people!
For now, I'm sticking with Greenseng.