Math Gym creates puzzles designed to exercise your mental arithmetic.
A series of calculations scroll up the page, and you have to work out the final answer. As you progress, the math gets harder and scrolls away faster, but you can always pause it for thinking time or to get a hint.
With 10 levels of difficulty (from simple addition and subtraction through to cube roots and primes), and 10 speed settings, there is something to challenge everyone from elementary school to math graduate.
At the harder levels the calculations are often designed to be canceled out by later ones. So it pays to look (and think) ahead.