KCAPTCHA is a free and open source PHP solution to generate human validation images (CAPTCHA).
KCAPTCHA is meant to be a very strong protected one but requires no special hosting features, only PHP with GD library.
Example (generates on fly, so refresh this page to see another variants):
How it works: script creates session and writes to it with name $_SESSION['captcha_keystring'] randomly generated text string. And outputs image with distorted text. While checking visitor humanity, you should compare code string from session with one from visitor input. Be sure that session contains non-empty code string!
System requirements: PHP v. 4.0.6 or above with GD2 library support. Not required font libraries like Libttf etc.
The package contains pixel fonts, so you should not search and install fonts.
You can customize colors, character sets, string and image size.
Download KCAPTCHA 2.0 now