Quento was recommended to me just recently by someone I met at Macworld or 360iDev, and unfortunately, I don't remember who told me about it. But the recommendation was great, and now I'm passing it on to you. As you can see above, Quento is a math-based puzzle game, where you use a grid of numbers and symbols to try and put them together into a target number.
The game's interface is actually half the fun -- like the popular word game Letterpress, Quento is very stylishly designed, and as you complete each target number in the puzzle, it gets replaced in the menu above. You can swipe the little menu right or left to go up to harder difficulty modes, or down to a "free play" mode (where you just get a number to make, and you can use any combination to make it).
The game is casual, obviously, but it's not necessarily easy -- at times, I found myself racking my brain to try and line the numbers up right. But there's always an answer in there, and no matter what your math knowledge is like, Quento does a great job of rewarding concentration and smart gameplay. You can pick it up now as a free, universal app.