Project shyknight is a classic-style platforms game like Manic Miner and some others from early 80s. It has very nice and polished, colorful retro graphics, and simple but addictive gameplay. Eric is in love with a greedy girl named Martina. He must collect all the items she wants to make her happy.
QMentat helps you practice mental arithmetic. It uses arbitrary-length fixed point arithmetic, and can handle any size numbers, only limited by the size of your screen (and in the case of division with an integer result, the speed at which the number can be factored). It is also quite configurable, allowing you to tailor the questions to your needs.