There are a few great examples:
- MVC reCaptcha – making reCaptcha more MVC’ish.
- ReCaptcha Webhelper in ASP.NET MVC 3
- ReCaptcha Control for ASP.NET MVC from Google Code.
This has also been covered before in this Stack Overflow question.
NuGet Google reCAPTCHA V2 for MVC 4 and 5