I have always wondered how easy or hard it might be to write a program that can crack CAPTCHAs (those messed-up-text thingies you have to decipher in order to leave a comment on certain blogs or get a new account on Gmail, such as shown here). Two researchers give a great account of how they cracked Yahoo's Gimpy CAPTCHA system at http://www.cs.sfu.ca/~mori/research/gimpy/. Their algorithm works over 90 percent of the time. I don't know how old their work is or whether Yahoo has changed its CAPTCHA system in the interim. But it is interesting work nonetheless.
It is interesting to reflect on the fact that even a CAPTCHA-cracking algorithm with a dismal success rate (say 10 percent) is still plenty good enough for spammers who need to be able to create bogus e-mail accounts by the thousands via crackbots.
Judging from the amount of spam that gets by my spam filters every day, I'm pretty sure professional spammers are creating better and better CAPTCHA-defeating algorithms every day.