Google reCAPTCHA Validator
Some quick shots at the code  


by retrieving the ReCaptchaLocationInclude property an ArgumentNullException is thrown, because you didn't initialize the ExtraClasses property. I would also like to suggest changing this property from autoimplemented to a normal one, so you can validate any set value.  
bool passedReCaptcha = false; is not really a good name here. so I wouldn't use this variable at all. Instead I would replace this  

if (reCaptchaResult.IndexOf("\"success\": true") &gt; 0)
    passedReCaptcha = true;  


with 

    return (reCaptchaResult.IndexOf("\"success\": true") &gt; 0);


and for the IDE's love add a return false; at the end of the method. If you don't want to do this, it is fine too, but you should replace the former if with passedReCaptcha = (reCaptchaResult.IndexOf("\"success\": true") &gt; 0);