I’ve found a great way to have fun with your linux server. First of all, install cowsay. If you use Ubuntu server or Debian, you can use apt-get install it: 

sudo apt-get install cowsay

Now, you have to wait for another user to connect to the server via ssh. You can use the who command to see who is logged in. With two open terminals on my Ubuntu box, who‘s output looks something like this.

tudor@thor:~$ who
tudor    tty7         2009-06-17 20:31 (:0)
tudor    pts/0        2009-06-17 20:36 (:0.0)
tudor    pts/2        2009-06-17 22:21 (:0.0)

Let’s assume that the victim is the one using the first terminal, pts/0. Now, all you have to do is punch the following like in your terminal:

cowsay -f tux "I see you" > /dev/pts/0

…and his terminal will look like this:

tux

Doesn’t this make you feel better about yourself?

Later edit: the party continues here!