Recently (and specially after Web 2.0 revolution) many websites started to use more complicated CAPTCHA solutions on their websites in order to prevent spam and bot attacks. There are many free solutions and tools for users and developers to implement CAPTCHA technology in their applications. There are also many ASP.NET controls and components for this approach but many of them are so simple to hijack or don't offer features that these days we see on professional websites (some features like reload function, voice, etc.).
A few days ago I found a link to reCAPTCHA on Yahoo!. reCAPTCHA is a free (but professional) tool for basic users and also developers. To use this service you should first signup, receive an API Key for your website (you can receive as many as you need) and use one of easy-to-install plugnins provided. And good news for ASP.NET developers is that you can download and use a server side control and enjoy reCAPTCHA with two lines of code! The API Key guarantees your website and prevents attackers collect answers from visitors and use them. There are three different themes to choose.
If you want to use reCAPTCHA on a Web 2.0 website you can use AJAX API just by adding this line of code to you html or webform file:
I strongly suggest you download and test this tool in your web applications.
Quick Links: reCAPTCHA.net | Live Demo | Why reCAPTCHA | Signup | reCAPTCHA.NET Control | Email Protection With reCAPTCHA