If you type a command on Linux that doesn't exist, normally you get this:-

alan@bishop:~$ mplayer
-bash: mplayer: command not found

With the package command-not-found installed (which is by default installed as of Ubuntu 7.04 [Feisty] [due out in April]) you get something like this:-

alan@tigger:~$ mplayer
The program 'mplayer' can be found in the following packages:
* mplayer-nogui
* mplayer
Try: sudo apt-get install
Make sure you have the 'multiverse' component enabled
-bash: mplayer: command not found

How cool is that!?

More Here [1].