Conkers has been a children's game here in the UK for donkey's years, I used to play as a kid. I've no idea how popular it is elsewhere.
Conker is a slang term for a horse chestnut.
In Autumn each year children would go around collecting them where they fell. The game is simple: drill a hole through the chestnut, then thread it onto a piece of string or shoelace. You challenge someone to a match, and take turns hitting your conker against your opponent's by flicking your wrist to swing it around. Your opponent has to hold the string and dangle his conker in place while you hit. The one who's conker breaks first is the loser.
There were all sorts of theories as to how to make conkers more resilient, I remember soaking some in vinegar to try to get the edge.
