4-Way Count Down
This is a game produced by Cadaco, and is suited for ages six and up. It can be played by two to four players, which makes it a good game for the family, but less adapted to the classroom. The game helps teach children basic math functions, such as addition, subtraction, multiplication, and division. The addition and subtraction are especially helpful in teaching children to countdown, since it requires a knowledge of the number sequence.
www.educationallearninggames.com
The Numbers Game
This is a game based on a game played on the U.K. show Countdown. It's designed to use in the classroom, and will help children who are learning to count up to and down from larger numbers. The highest number in this game is 999, but you can set the program to whatever limit you prefer. The object of the game is for students to use a given set of numbers to reach a target number, using addition, subtraction, multiplication, and division.
csfsoftware.co.uk
Mad Money
This is a game provided by PBS which not only teaches kids to countdown, but teaches them financial responsibility as well. The object of the game is to choose one of the provided objects to "save up" for. The game gives you an allowance, and you have 30 "days" to save up enough to purchase your item. Children can count down the remaining days they have to save up enough, as well as calculate how much they have to save per day to reach their goal.
pbskids.org