Addle is a two-player computer game (or the computer can be the second player, effectively making it a one-player game), played on an 8 by 8 square board. Each square contains a number (a positive integer between 1 and 16 inclusive). The game starts with one (randomly chosen) column of the grid highlighted. The first player chooses a square in that column, and the number in the square is added to that player's score. Then, the row of that square is highlighted and the second player chooses another square from that row. That number gets added to the second player's score, and that column is highlighted, and it's the first player's turn again. A player may not choose a square which had previously been chosen (the chosen squares are displayed differently to make it clear they cannot be chosen again). The goal is to beat your opponent's score (by as much as possible).
The rules are simple, and the game is fun. It teaches how to plan ahead; choosing the highest number in a column may highlight a row with an even higher number, so that move would probably be worse than choosing a lower number which is the highest on its row. The more moves you can think ahead, the better a player you will become. The implementation I have of the game, psAddle, is available for free download for the Psion series 5mx handheld computer, and is distributed by FreEPOC (the operating system of the Psion 5mx is called EPOC). psAddle allows you to play the computer at 3 levels of difficulty, or allows two people to play each other. psAddle was first written by John Peat, and is now maintained by Martin Guthrie.