My company manages a number of sites and if their servers or firewalls go down we need to know, what we'd really like is something that pings the host every few minutes and shows on screen if they are up. I'm considering writing it but there must be something simple out there that does this already, maybe my google-fu isn't good enough.
If you don't want to do the setup yourself (not to mention the "who watches the watchmen?" problem with self-hosted Nagios etc.), something like Pingdom should do the trick if the servers are accessible over the public internet.