Google Image Labeler
Google has an ingenious little game on their site now called the Google Image Labeler. Here's how it works:
You are paired up with another site visitor who is also willing to play. You are then each shown the same image and asked to come up with keywords that describe the contents of the image. Woman, brunette, sunset, beach, etc. The idea is to come up with matches - words that both you and your partner thought of.
Google is using this game as a way to tag images and use those tags to improve image search results. Because the more often an image receives the same descriptive keyword, the more likely that word is a good description of the image. It's a brilliant way to harness the power of the masses and make the mundane and immense task of image tagging fun.