Network connection down? Want Mac OS X to announce to you when you are back online? Turn up the volume; start Terminal (by clicking on the Spotlight magnifying glass in the upper right of your screen, typing 'terminal', and hitting Return); type or paste the following into the window; and hit Return:
This will speak the words "Back online" to you when your network connection is reactivated.
Why this works
while true; do <some command>; donewill repeat the specified command until it hits a break statement
ping -c 1 <some IP address>will send one packet to the specified IP address, returning true (if the server responds) or false (if the server cannot be reached or fails to respond)
18.104.22.168is one of Google's public DNS servers, which should always be online and always respond to pings
say "<something>"will use the built-in text-to-speech synthesizer to speak the given text aloud and return true
breakwill exit the infinite loop
&&chains commands, executing each only if the preceding command returned a value of true
Putting these together, this command will send one packet at a time, forever, to a server that should always respond, and the first time the server responds, it will tell you that you are back online and stop checking.