Human computation

In traditional computation a human can give certain tasks to a computer to perform by providing a formalized problem description.
Human computation is a process in which the computer "outsources" particular tasks to humans. There are many problems which can be solved better by humans than by the most sophisticated algorithms. Some bold examples are solving capcha, or labeling images with words.

I heard a great presenation given by Luis von Ahn on human computation.
This can be found at: http://video.google.com/videoplay?docid=-8246463980976635143