http://jcaptcha.sourceforge.net/
JCaptcha -
Last Published: 01 Sep 2012 | Version: 2.0-alpha-1-SNAPSHOT
jcaptcha.sourceforge.net
https://developers.naver.com/docs/utils/captcha/
캡차 API 개발가이드
NAVER Developers - 캡차 API 개발가이드
developers.naver.com
http://simplecaptcha.sourceforge.net/
SimpleCaptcha - A CAPTCHA framework for Java
SimpleCaptcha is a Java library for generating CAPTCHA challenge/answer pairs. SimpleCaptcha is intended to be easy to implement and use sensible defaults, while providing easily-accesssible hooks for customization. Example implementations are provided in
simplecaptcha.sourceforge.net
https://developers.google.com/recaptcha
reCAPTCHA | Google Developers
Protect your site from spam and abuse.
developers.google.com
다양한 Captcha API 인증 방식을 비교할 수 있는 사이트
http://caca.zoy.org/wiki/PWNtcha