While waiting for one of the Twitterfall servers to come back online after an air conditioning failure in the datacentre, x5315 and I realised that instead of keeping an eye on a ping command, we could be told when the server was back online with this really simple (Mac-compatible) command line:

ping -o server && say "the server is back up"

Also, if you use Prowl on your iPhone, you could use something like this instead:

ping -o server && curl -q --insecure \
"https://prowl.weks.net/publicapi/add?apikey=YOURAPIKEY&application=Server Monitor&event=The server is back up&description=Server up"

And when the server is back up, you’ll get a push notification on your iPhone.

Simple, but handy.