Ninja
Source: A beer garden following (the final iteration of?) TIGJAM UK. I can't remember who taught us all the rules.
Gather a group of players. Everyone stands in a circle. The game starts with everyone chanting "3... 2... 1... Ninja!", at which point everyone strikes a ninja pose.
From there, players take turns, and play progresses round the circle. When it is your turn as ninja, your goal is to tap someone else on the back of the wrist, knocking them out of the game. This may only be done as part of a single, fluid movement, however. Making more than one distinct motion will see you knocked out of the game.
The attacking ninja can move any part of their body when attacking. If, on the other hand, you are being attacked, you can defend yourself by trying to dodge their attack. Defending ninjas are only allowed to move from their waist up, however.
If you lose your balance or fall over during play, you are out of the game.
Play progresses until there are just 2 ninjas left. At this point, the 2 ninjas re-position themselves to stand back-to-back, for the final duel.
Variants:
T-Rex Ninja
Source: TIGJAM UK
The same rules as above, only players must keep their elbows clamped to their sides, and are only able to move their arms from the elbows down.
Elephant Ninja
Source: TIGJAM UK
The same rules as above, only players have to hold one arm against their head like an elephant trunk. The other arm does not come into play.
Duo Ninja
Source: Abertay folk games workshop: Lynn Love
Requires an even number of players
Players pair up, and link their left arms. One player faces forward, the other faces back. Otherwise play follows the above rules. Although players are paired up, they are not working together, and can attempt to knock out their partner. When a player has lost their partner they continue playing as normal. The winner is the last - single - player left standing.
Frog Ninja
Source: Abertay folk games workshop
Players crouch on their haunches like a frog and have to stay that way throughout play, hopping if they need to close the distance to other players as attackers. Otherwise, play follows the above rules.
Blind Ninja
Source: Abertay folk games workshop
Players keep their eyes closed and attempt to knock out their opponent following standard Ninja rules. This is safest played with just 2 players and spectators on hand to prevent any accidents.
Blind Cat Ninja
Source: Abertay folk games workshop
The same rules as Blind Ninja, only this time, all player movements must mimic the movements of a cat, complete with appropriate meows, hisses, etc. The "3... 2... 1... Ninja!" chant becomes "3... 2... 1... Cat!".