Have you ever tried out Google Image Labeler? I had never heard of it, and I'm not sure if it's new or not.
Doing a quick image search on something, I was presented with the question at the bottom of my results asking whether or not I wanted to try it. Curious, I clicked the link to find out what it was. According to their web page, Image Labeler is:
"...a new feature of Google Image Search that allows you to label random images to help improve the quality of Google's image search results."
So you get paired up with a partner and for two minutes you are shown a variety of images. As each image is presented, you (and your partner) enter plausible labels for the image. When you and your partner match up on a given label, you move on to the next picture. At the end of it, you're presented with a score and ranking.
It's no surprise to me that Google has given a neat way to help improve their search product and involve the user in the process. They don't hide the fact that yes, they are using you to improve their search. I took a few minutes out to do it and it was quite fun to see how high I could get my score. It didn't bother me in the least. How effective it is at improving their product I don't know.
[note: I'm assuming you need a google account to use Image Labeler - not entirely sure]