I developed this little site as a simple math challenge game. Some of the elements here are things I’m experimenting with using on other projects.
Essentially, the game offers a choice of four basic arithmetic operators to choose from, then generates math problems for you to solve. Correct answers add to your score while incorrect ones subtract, and your score is displayed both numerically and as a meter leading to 100 points. The numbers generated are multiplied by a factor of your score, so in general the problems become progressively harder as you solve them.
Part of my interest in developing this was to build something that had undefined limits, so there is unlimited potential for users to compete with each other to achieve the highest possible scores.