Captcha Alternatives (in Spanish)

http://www.beoninternet.net/honeypot-anti-spam-sin-captcha/
http://infoaccess4all.wordpress.com/2012/11/12/captcha-cuando-la-seguridad-atenta-contra-la-accesibilidad/
http://pixelar.me/tip-sencillo-para-evitar-el-spam-en-formularios/
http://www.facilware.com/seran-asi-los-nuevos-captchas.html