Key features: generates CAPTCHA image with textual code rendered in it, full server side component, it is not dependent on other software and it can be royalty free distributed. Also it includes high-quality samples, free 24x7 support and FULL SOURCE CODE.
(CAPTCHA is an acronym for "Completely Automated Public Turing Test to Tell Computers and Humans Apart". As the name suggests, it's a test to distinguish the degree of being human. As defined on the CAPTCHA home page at the Carnegie Melon University School of Computer Science's Web site:
CAPTCHA is a program that can generate and grade tests that: Most humans can pass. Current computer programs can't pass.)