Capthas are becoming less and less effective, as methods to overcome them are being developed. The best system is RECAPTCHA which uses words from old books that are hard to read for recognition systems by themselves so by recognising the captchas you also help digitize the books. Still the captcha system has one big exploit – human factor. What the spammers usually do is setup a system on a pornography site that takes the captchas from a protected system and requests the uses to input it for example to “see the next image”. Thus a single porn user can enable sending of hundred of spam emails, and digitizes old books at the same time 😀
