If you are interested in a good text book on Information Retrieval a.k.a Search, here is one,

http://www-csli.stanford.edu/~schuetze/information-retrieval-book.html

It’s online and oh yes you can’t beat the price!

I thought the captcha system for identifying humans and blocking bots was cool ! recaptcha is even more cooler. The image [challenge] presented has two words instead of one. They use the ”other” word to digitize books scanned using OCR technology. The problem with OCR is that it does not recognize all words but the nice thing is that it can tell you so. The “other” word is now presented to a human to be interpreted.

Check them out here recaptcha ! I give them a thumbs up.