https://github.com/inorganik/ImagelessCaptcha
How is three-hundred and seven written as a number?
https://www.phpcaptcha.org/try-securimage/
可以听语音的验证码
https://visualcaptcha.net/
图像选择和语音验证码
https://github.com/vincenting/campto
汉字和数字结合计算的验证码
http://www.josscrowcroft.com/demos/motioncaptcha/
画图验证
https://jessekorzan.github.io/rUhuman/
滚动条计算验证码
https://github.com/ARCANEDEV/noCAPTCHA
简单粗暴，基于谷歌，国内无法使用