pongClock is all you need when it comes down to Pong on the Dashboard. If idle, it plays Pong against itself, while the score always displays the current time. But if you feel competitive, you can always play Pong against the widget. Start the game by pressing the racket icon bottom-left. Press the Up-arrow to move the right player upwards, the Down-arrow to move it downwards; well, that's too obvious. You may pause the game any time by pressing any other button. The speed of the game increases, if you win, and decreases, if you lose. The player who reaches 10 points first wins.
To really enjoy the good old Pong-feeling you can play fullscreen. Just click the toggle-icon beneath the racket icon and there you go. Same applies to get out of fullscreen.
There are a few settings available, like turning off the reflective glass and increase CPU usage to get a smoother game. You can also adjust the transparency of the widget for fullscreen mode. However, setting this too low may give you problems getting out of fullscreen because the controller does not show up; if that happens just hide and show the Dashboard again and the controller should be visible again. If nothing helps, press Apple + R to reload the widget and all will be fine.