Unfortunately, the spammers used those to overload my mailbox.
Solution - use form mail to hide the email address.
And the spammers figured out how to use that to send crap - but no where near as much.
In 1997, various groups began developing methods (such as CAPTCHA's) to protect all types of online data submissions, including form mail.
For low volume sites, this is all you need.
If you still have a problem, and you think a program is decoding the CAPTCH, the text can be distorted and placed on a confusing background.
For high profile sites, nothing will work for long. For instance, some places the labor is so cheap that the spammers can afford to hire people to decode the CAPTCHA's.
Free formmail with CAPTCHA ($36 extra) written in php.
How to modify a form to require the same word every time - if the user does not type in the correct word, the form will not be submitted.