File this one under brain training games: Math Mountain is an addictive arithmetic game wherein you climb a mountain, competing against another person or the computer, by answering math questions correctly.
It's only BODMAS (without any brackets - Happy Bodmas!), so you don't have to worry about remembering high school algebra, and the questions vary from very easy (1 / 1 = ...) to fairly hard (123 - 75 = ...). You may be able to answer the questions easily, but you are being timed and you also have to choose the correct answer from a multiple choice, which, believe it or not, makes it harder. (A few times I've worked out the correct answer quickly, but the mouse slipped and I selected the number next to the one I wanted.) In the meantime, you're racing the CPU to the top of the mountain and collecting power-ups such as the ability to stop the opponent from moving or to remove all but two of the multiple choices. If you answer incorrectly or not at all, you'll slide backwards, proportionate to the difficulty of the question.
As you play through the levels, your side of the mountain will get longer and less steep, so that you have to answer more questions faster than the CPU. You'll have a larger number of multiple choices, making the chances of quickly clicking the wrong answer higher. However, if you do lose, you have a number of lives, so you can opt to "Continue" after the level has been lost. When you lose, you get to keep any power-ups you haven't used for the next life, so I suggest you save power-ups as much as you can—if you know you're going to lose badly, you may as well save that Stop power-up for the next life!
If you're not very good at arithmetic, Math Mountain is a fun way to practice; if you're already good, then why not give your brain some exercise? You can even turn on "Unlimited Mode" in the options by deselecting "timed" but that disqualifies you for achieving a high score. I haven't tried the multiplayer version, so if you give it a go, tell us about it in comments!