love running a drill called "ADD-A-MAN". One team starts with one player and the ball. That player sprints down the pool and takes a '1 on nobody shot' on the opponents goalkeeper. As the shot is in progress, the opposing team sends two players that are on the 5m line on a counter attack against the single player who just shot. This will create a 2 on 1 player advantage for the shooting team. Once they take their shot, the opponent sends two more players into a counter attack which creates a 3 on 2 opportunity. The process continues until both teams are at full strength and able to get a full strength counter attack.
The drill is excellent for working on counter attack options and fundamentals as well as the occassional 3 on 2 or 4 on 3 opportunities that present themselves in actual games. It is also a great drill to work on conditioning.