There are approximately 3.8 billion improv games currently in existence worldwide. Many of these games are played by Uncalled For regularly. Useful lists of improv games can be found here and here.
Here is a list of games that Uncalled For prefers to play offstage:
Played like regular poker, but without any real money or chips. Instead, bets and raises are made using objects and concepts that players do not actually have legal ownership of, such as the world supply of rubber, the laughter of children, or Sweden.
An endurance challenge to see who can balance a book on their head for the longest while performing various tasks, i.e. performing in an improv scene, tying their shoelaces, strolling down the boardwalk, etc.
For two players at a time, though often played as a tournament, with winners of respective challenges facing off against eachother. Players stand only on one foot, and when ordered to do so by the referee, begin hopping around, still on one foot, attacking eachother. The first player to allow any part of their body other than the designated foot to touch the ground loses.
A co-operative billiard game for two or more players, the object of speed pool is to sink all the balls on the table, in any order, as quickly as possible. Turn rotation works just like in regular pool, except stripes or solids (or even the 8-ball) become irrelevant and it only matters whether or not you sink a ball or if you scratch.
Just like playing catch with a frizbee, football, tennis ball or anything else, except there must be at least 4 objects in play simultaneously. Catching all objects without passing or dropping any of them is called "Hitting the Big Time" and gives one instant celebrity status.
Two players stand on either side of a circle. They are each given a superball on the end of a string. The first player to cause the other one to choke to death on the superball wins. These are the rules to Distance Jump.
This game has become quite popular since Uncalled For invented it, you can learn more about it here.