- Most popular is recaptcha by google
- Honeypot technique – include a hidden field, if it is filled, then it is a robot
- A simple random questions. E.g. is fire hot or cold , five – 3 is ___
- http://nomorecaptchas.com/ (with price)
- https://visualcaptcha.net/ (no longer developed but they claim it still work)
- Algorithm
- after 5 failed attempt, lockout IP for 15 minutes
- if 5 times lockout, lockout IP for 24 hours
- if 3 times lockout, blacklist IP permanently (option for sys admin to clear)
- More alternatives