web server, ftp server, counter strike server, irc server?
I mean someone could write a mod to ping servers and update an 'online page' every minute or so, but that doesn't tell you if a specific daemon's running or not.
for instance for the webserver you'd have the script load a URL and if the call fails mark that site as down, for an ftp server you'd do something similar... for a game server or IRC server the script may need to check socket level.
btw, I'm not saying I'll write this hack (6 of them on my todo already at the moment), but I think the modders could do with some extra info on this