Wallball is an excellent sport popular among elementary kids, particularly in the southern United States.
The rules of the game are simple. There can be one or more balls in play, and all players stay within a relatively small area (maybe 30 square feet) with a wall in front of them. The ball is then thrown at the wall as hard as possible. When it rebounds, the player who catches it is the next thrower. Play is fast and furious, especially when multiple balls are used. If a catcher drops the ball on the catch, or a bad throw results in a complete miss of the wall, the player must run up and tag the wall, during which time he or she is fair game, and may be pegged without penalty.
More advanced variants may include a point system for the number of times a player is pegged, or for the number of times a player drops the ball. Players may also be forced to sit out when pegged, or even trade out with other players on the sidelines, if there are too many people to play at once.
Equipment consists of merely the ball and a wall. The game is well suited to be played outdoors, where the side of a building can make an excellent wall. A concrete flooring is best, but grass will make do. The ball itself is usually palm sized, maybe 3/4 the size of a baseball, and rubber with a hollow core. Racquetballs or tennis balls make decent substitutes according to some. The idea is that the ball is somewhat heavy, rebounds quickly, and causes a moderate amount of pain on impact with the players.
The beauty of the game is in its simplicity. Recess begins and ten kids grab a few wallballs and run for the nearest wall. As was the case in my elementary school, isolated outdoor concrete sheds make for multiple possible games going on at once, one against each side of the building. The game takes no real skill, and causes no real damage, but has enough elements of both sport and warfare that it can entertain a maximum number of kids with a minimum of effort.
A few final words on the social aspect of the game are in order. Games, especially when run concurrently in close quarters (as in the shed example provided above), often interfere. A player who drops the ball or is hit as a result of a foreign ball does not have to tag the wall. Another important rule is the ability of the majority on the court to limit the number of players in the game. If a new player shows up, it is up to the majority of the current players to decide whether or not the newbie shall be admitted.
You now know everything required to start up your own games of wallball!