There was once a time when Macs were pretty well regarded for their games, and Crystal Quest was their Tetris -- engrossing, addictive, and impossible to beat. You used the mouse to control a little spaceship that looked like a hockey puck, collecting all the crystals in each screen-sized level. Once you had all the crystals, a gate at the bottom of the screen would open up, permitting you to escape into... a slightly harder version of the level you just played, with more types of enemies, more land mines, and more crystals you have to pick up. This masochistic treadmill of impending doom was memorable enough to its players that they eventually ported it to the Xbox Live arcade, with (optionally) modernized art.