Recently I needed to improve CAPTCHA on a website. Here are the different types of CAPTCHAs I found.
- Web Service (Google reCAPTCHA)
- Locally-Run (Securimage PHP CAPTCHA)
- CSS (Honeypot / Invisible) CAPTCHA
- Video CAPTCHA (NuCAPTCHA)
- Math CAPTCHA (generate 2 numbers to add or something else only humans can evaluate and have them enter the correct value in another field)
Personally, the CSS CAPTCHA is the most elegant since users won’t even see any CAPTCHA field.