Lifemapper
Someone mentioned this on urban75. It's like the SETI project, but instead of searching for little green men, it maps species across the globe and projects their distribution (I think that's what it does). Anyway, the chap who originally mentioned it has set up a U75 group, so I've decided to join up and put my computer to some good use. So far I've completed three units in four and a half hours. lifemapper.org