These are just a few steps, as the latest reCAPTCHA library has all the pieces you need for ASP.NET MVC:

Go to Get reCAPTCHA, which gives you the public and private keys you need to use reCAPTCHA with your site.
Download the latest .NET library at: http://code.google.com/p/recaptcha/downloads/list. Currently: dotnet-1.0.4.0.
Add <%= Html.GenerateCaptcha()%> to the form you want to protect with the reCAPTCHA. Naturally, you have to add the namespace for the extension to be recognized, some of the options:
- Add the namespace to the web.config:
```
<pages>
      <namespaces>
...
        <add namespace="Recaptcha" />
```
- Add the namespace at the view:
```
<%@ Import Namespace="Recaptcha" %>
```
- Add your own extension method that wraps it. I changed the signature to return MvcHtmlString to prevent double encoding when using it with “<%:” instead of “<%=”
```
public static MvcHtmlString GenerateCaptcha(this HtmlHelper htmlHelper)
{
    var html = Recaptcha.RecaptchaControlMvc.GenerateCaptcha(htmlHelper);
    return MvcHtmlString.Create(html);
}
```

Add the RecaptchaControlMvc.CaptchaValidator attribute to your controller. Also add parameters named captchaIsValid and captchaErrorMessage. Just like:

[RecaptchaControlMvc.CaptchaValidator]
public ActionResult MyMethod(Something else, bool captchaValid, 
   string captchaErrorMessage)
{
// do something if (!captchaValid)
}

Configure your keys. Some options:
- Add to appsettings in the web.config, with entries named: RecaptchaPublicKey and RecaptchaPrivateKey
- Set at Application Start:
```
RecaptchaControlMvc.PrivateKey = privKey;
RecaptchaControlMvc.PublicKey = pubKey;
```

Adding reCAPTCHA to ASP.NET MVC