1. reCAPTCHA

2. Securimage

3. WebSpamProtect

4. Cryptographp

5. CAPTCHA-Service
captchas.net provides CAPTCHA images and audio files you can use in html-forms. To use this technology, your web pages have to be generated dynamically in any programming language PHP, ASP, Perl, Python, JSP, Ruby.
6. WP Captcha-Free

7. ProtectWebForm

8. ProtectWebForm
OpenCaptcha is a simple web service which requires no special configurations or modules. Basic installation is cut-and-paste, and requires no ability to program image manipuation scripts. New fonts, image algorithms, and distortions applied weekly.
9. Form-to-email script protected by Captcha

10. freeCap
freeCap is a GPL CAPTCHA script to stop spam. It has been used on any form on any kind of website. It does require some knowledge of PHP to install, though there are several captcha plugins for forum and blog software listed below.
For the Normal JSP implementation (Which you can try):
http://captchas.net/sample/jsp/
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