Useful: Recaptcha

Most websites use captchas somewhere. A captcha, for those who do not know, is a system used to confirm the form is being submitted by a human and not an automated computer program. There are various types of captch such as math and graphical. For the math you have to solve a simple math such as "3 + 7".  For the text you are asked to type in the text you see on the screen.  The text is usually distorted to make it difficult for computer programs to decipher them.

Despite the common misconception that captchas are not accessible, many are. The match captcha is usable by all out of the box. Many of the graphical one also provide an audio version. There is also the misconception that you need an insight into computer graphcs to use the latter, but this simply is not true, as long as you can read you can use it.

The captcha system I use on many system is offered, free, by Recaptcha.net. This offers a graphical system with audio making it accessible to all including the visually impaired. This is the main reason I use this system but also that it helps them to digitise books. Learn more about this on their about page.