Uses old Android phones to listen for chainsaws. EcoHack 2014 project added realtime SMS alerts & app for park rangers, as well as an impressive dramatization of intended usage.
Shift your energy use to times when it's green. WattTime started at EcoHackSF 2013, and used EcoHack World to add support for water/carbon impact of energy generation.
Nerds for Nature - Monitor Change TimeLapser
Monitor recovery from Mt. Diablo fire using crowdsourced photos. This project modified OpenCV to stitch various photos together into a time-lapse image.