Not really a game as such, but a great way to get pissed quickly.
Basically, whenever your pint is fuller than another person's, you can challenge them by asking them "Do you know the rules of the game?". They must then reply "It would be rude of me not to." The challenger then tosses a coin, and the challenged person calls heads or tails. If you call correctly, the challenger must finish his/her pint; if they are right, you must finish yours.
Any deviation from these rules, ie not responding correctly, dropping the coin, or challenging someone who has a fuller pint, results in the transgressor having to finish their pint.
Note that it is not wise for one person to challenge everybody else in quick succession, or they will be in a very sad state very quickly (pay attention, Jock?)