Reddit account creator
Some points about your code  


extract the code to create an account to a separate class. In this way you can reuse it without problems. This can even be a static method.  
variables should be named using camelCase casing. See the naming guidelines.  
as you don't use neither the TemporaryCookie nor the CreateAccountCookie you should remove these varaibles.  
the assignment of POSTRequest.KeepAlive = true; is superflous because the default of the KeepAlive property is true.
string POSTRequestCode = POSTRequestReader.ReadToEnd(); is superflous because it is never used.  
because Stream is implementing IDisposable you should enclose it in a using statement. This will take care that the Stream is closed and disposed.
you don't use the HttpWebResponse POSTResponse so you can remove it too  


Implementing the mentioned points will lead to:

private static string postDataToFormat = "op=reg&amp;dest=https%3A%2F%2Fwww.reddit.com%2F&amp;user={0}&amp;passwd={1}&amp;passwd2={1}&amp;email=&amp;api_type=json";
public static void CreateRedditAccount(string userName, string passWord)
{
    string postData = string.Format(postDataToFormat, userName, passWord); 

    UTF8Encoding Encoding = new UTF8Encoding();
    byte[] postValue = Encoding.GetBytes(postData);

    HttpWebRequest POSTRequest = (HttpWebRequest)WebRequest.Create("https://www.reddit.com/api/register/" + userName);
    POSTRequest.Method = System.Net.WebRequestMethods.Http.Post;
    POSTRequest.ContentType = "application/x-www-form-urlencoded";
    POSTRequest.Referer = "http://www.reddit.com/";
    POSTRequest.UserAgent = "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36";
    POSTRequest.ContentLength = postValue.Length;

    using (Stream POSTRequestStream = POSTRequest.GetRequestStream())
    {
        POSTRequestStream.Write(postValue, 0, postValue.Length);
    }

}