On many sites, the forms are accompanied by an insert request to copy a randomly generated code.
Such a system called CAPTCHA, used to limit abusive automated mailings (SPAM).
The following sites offer free tools in PHP, to put such a system on its website:
- CryptoPHP, Free, PHP> = 4.3.0. Available in plugins for WordPress, wpMu, Symfony and Guppy.
- ReCaptcha, Free, operates Web Service (online)
- WebSpamProtect, Fee, PHP, Perl, ASP or ASP.NET
- WP Captcha Free Free shows no code to copy, it is based on a system of self-generated hash making it impossible to add automated comments
- ProtectWebFormis an online service offering a captcha, coupled to an audio system (for blind persons)