I recently had a requirement to implement a CAPTCHA for one of our contact forms to minimize spam.
After much research and consideration I decided to use reCAPTCHA and the reCAPTCHA ASP.NET library.
Step 1 - Create a free account
http://www.google.com/recaptcha
Step 2 - Download the reCAPTCHA ASP.NET library
http://code.google.com/apis/recaptcha/docs/aspnet.html
I'm using the .Net control but as you'll see it has plugins for several platforms.
To get it working was just a matter of referencing the library in my project, dropping the control on my page and referencing the public/private keys generated in step 1.
After much research and consideration I decided to use reCAPTCHA and the reCAPTCHA ASP.NET library.
Step 1 - Create a free account
http://www.google.com/recaptcha
Step 2 - Download the reCAPTCHA ASP.NET library
http://code.google.com/apis/recaptcha/docs/aspnet.html
I'm using the .Net control but as you'll see it has plugins for several platforms.
To get it working was just a matter of referencing the library in my project, dropping the control on my page and referencing the public/private keys generated in step 1.