Work for Google, without pay?

I recently saw an interesting option with Google images. Google is actually asking its user base to go through and tag images in its directory so that is has a very accurate, human moderated title system to deliver more relevant results to all those jpeg searchers out there. Work without compensation seems like an interesting way to cement your position at the top. Apparently the execs at the Googleplex felt it was about time to abuse all of its loyal followers as to keep total domination?