To have your very own signature like this, you will have to sign up and register at www.l33tsig.net. Once you have created an account, you will need to download and unzip the client:
You will now need to add execute permissions on the setup application, and on the daemon:
chmod u+x l33tsig l33tsigd
Now you are ready to set up what you want to be displayed on your signature. Run the l33tsig file (either double click on the file, or run ./l33tsig within the directory in the terminal). You will need to enter your login details to begin with, and then you can select what you want to be displayed. When you have finished, run the l33tsigd, and login to www.l33tsig.net.
To make life a little easier, add the daemon application to launch at start up. To do this, go to System -> Preferences -> Sessions. Click Add, and browse for the l33tsigd file.