The same group (http://grassrootsmapping.org) which organized a citizen mapping of the BP oil spill is offering their DIY balloon-mapping kit as a reward in their Kickstarter campaign, which ends on January 30th. Having formed an DIY, open-source environmental science movement called Public Laboratory, they're now making open source hardware & software, and have grown to a movement of hundreds of people. $85 gets you a balloon kit, and their open source web-based map stitching software helps you convert your aerial images into an online map. There's even an infrared hack to take aerial infrared photos: http://publiclaboratory.org/tool/near-infrared-camera

Link: http://www.kickstarter.com/projects/1775485688/balloon-mapping-kits