Penny Farthings for rentThe SETI Project is well known, especially its @home effort which essentially is a huge distributed computing effort that aims to make effective use by stealing some CPU cycles from temporarily unused computers to do bits of computation.
There is yet another extremely large pool of processing power of which only about (or so we're told by shady books on the subject) 10-20% is used. That's obviously the collective brains of adult humans (we'll leave out children for they seem to putting it to use by playing and learning).
Perhaps our brains could be tapped from time to time while we're gossiping, watching inane programmes or films, staring at visuals and consequently not doing any conscious work. Maybe they could be used even while we are working, if it could be non-intrusive. Surely, this thought isn't practical today and will easily be controversial ;-) (don't want you peeking into my neurons & synapses, thank you). But the worst scenario of all would be if statistics about what we do with all the power available at our disposal thanks to our massive mom's-womb-endangering brains were made available. D'oh!