A friend recently sent me a link to a site called TinEye which is a reverse image search engine. The idea is you upload an image or paste in the url of an image and TinEye searches for that same image across the web. Although it can't access every website it is a handy tool for anyone interested to see how their photos are being used (especially if you've licensed some photos with a creative commons license and would like to see what people are doing with them). It could also be useful if you're worried about your photos being used without your permission, although all TinEye can do is point you to where the photos are being used so you can choose whether or not to take any action.
I tried TinEye out with a couple of photos I have uploaded to both Flickr, iStockphoto,and various blogs, giving the search engine the Flickr urls and it found the photos in iStockphoto and the blogs as well. TinEye says it's the first image search engine to use image identification technology rather than keywords or metadata so in theory it should be more accurate than other similar engines. It's free to use and there's no need for registration (although you can register for a free account if you wish which gives you a few benefits). You can try it out here.