Hacker Online is a nice game based on simulated hacking sessions, on a complete simulated OS. You have the chance to find the hacker hidden in yourself. You can fight against all the other users to stole passwords, information, data, images, and whatever you can.
But when you don't want to hack someone, you can use your terminal as a real computer, listening to music, watching videos, pictures, and playing some simple games. You can also message with your friends or post what you want on your Facebook wall directly from the terminal.