Wren - Offset your carbon footprint by saving rainforests
Klima - Go carbon neutral
Capture - A great free screen capture utility that allows you to capture either a window or the desktop and save it to either a file or the clipboard.
Website Carbon Calculator - Find out the how much CO2 your website emits
Aerial - A beautiful aerial screensaver for your Mac
YAYZY - Track the carbon footprint of each purchase in real-time