This one I ruthlessly swiped from flemco... hey, I live dangerously.
Game type: flash brain game
Time demand: fairly short
What it is:
It's fairly simple, yet at harder levels still surprisingly difficult. The simple idea behind this game is that of displaying a letter, shape, object... whatever... from the top down, one line at a time. Almost as if you had, say, a small slit in a sheet of paper, then someone moved the object in question (or a silhouette of it) slowly up behind the paper, so that you would see the top bit, followed by the next, followed by the next... but never all at once, in a cohesive shape. You instead have to guess at the letter, number, or whatever, based on the lines you see in turn.
And... that's pretty much it, really. It's a simple idea, but pretty delicious as you get into it.
Give it a shot!