A set of Nagios plugins I consider useful for myself.
Right now, you'll can find (with its further documentation):
Check if any of IP's belonging to your mail servers are included in Microsoft Live black lists. If you're blacklisted you won't able to send any e-mail to @hotmail.com addresses, @live.com and so. Microsoft has their own black lists, their rules and their policies, so can't be checked by a third-party.
Check if some defined string is present in a given URL. It can be useful together with other classic web checks, such as checking if port 80/TCP is open, if Apache/Nginx/ whatever HTTP server is running, if the right virtual host is enabled and the web server returns another thing than a HTTP code.