Sometimes you exec some long-running command in the shell and don't want to stare at it until it's done. Maybe you want to browser twitter instead? Try this:
echo -e "\a"
This command rings the internal bell sound on most shells and lets you know when the previous command is done. For example:
grep -r "some_string" /some/huge/dir ; echo -e "\a"
If you're on a Mac and the command completes while the terminal is in the background you'll also get a little badge and a jumping doc icon.