Human Pictcha

From ZZEE:

Human Pictcha is a protection in a form of an image (a captcha), which can be embedded inside your Web forms, and which will filter out various forum spam, guestbook spam, form spam, signup spam, password attacks. For PHP or Perl forms, easy to setup and maintain, full open source, compatible with the typical Web server setup, secure and hard to defeat by OCR. You can tune it to match your Web site looks has complete documentation.