Splitter is a computer game played on a square grid by two to four players. Each player has a supply of marbles of one colour (different from all the other players' colours). Each player moves in turn, as is standard in games. A move consists of a player placing a marble on a square. If the marble brings the square up to its 'threshold value', then the square 'explodes'. The threshold value of a corner square is 2 (there are 2 squares touching any corner square), the threshold value of an edge square is 3 (there are 3 other squares touching it), and the threshold value of the rest of the squares is 4 (there are 4 squares touching each of them). When a square explodes, one of the marbles is moved to each of the neighbouring squares - the exploded square is left empty. (Note that the total number of marbles on the board does not decrease - they are just scattered.) If a square with white marbles explodes, any marbles on a neighbouring square which aren't white are changed to white, and likewise for all the other colours.
Now, if a marble explodes off one square and lands on another, and brings the number of marbles in the new square to its threshold value, then it causes another explosion in that square too. This allows for very long chain reactions which can totally change the face of the board. One marble can change the board from 8% black, 88% white to 98% black, 1% white; this is what makes the game such fun. A player is out after he has moved at least once, but has no marbles remaining on the board (due to opponents' explosions). The game is won by the last person playing, after all the others are out. (It can be proved that there will always be a winner; the number of marbles increases each move and inevitably there will be a chain reaction which covers the whole board sometime if the game is not won before then.) The size of the grid can be chosen, so it can be a short game or a long game if you like.
Splitter is available as a free download from CarrotSoft for the Psion 5mx handheld computer. It has an option for the computer to play, so it can be played as a one-player game too.