This should sometimes be left blank! Fill in the email address of the Webmaster that runs this site, but ONLY if the webpage in question has a link or message saying that people can Email them.
Note: If you click a link on their page that says "Email me" and their address pops into your mail program, that's fine. It came from their website.
On the other hand, if it's not on their webpage please leave this blank! Yes, there are other ways to track down the webmasters email address but please don't do this. Spam is bad! :)