Despite being spoiled on the fancy graphics of next gen consoles I'm still a fan of old school games like Tetris. Its something that has withstood the test of time and that everyone is familiar with. You see people play it on their cell phones and almost every year a new Tetris game is released with some twist that doesn't really do well with players.
The original and best. Colorful blocks in different shapes raining down and your objective is to stack and organize them so they fit flush, making rows when net you major points. If you're unable to stack correctly and make complete rows, your accumulated blocks grow and if it grows too high, game over for you.
In this one you can select your level of difficulty and if you make it on the high scores, you see how you rank against players from all over the world who also play it.
Its free, fun, addictive and all I did to find it was Google 'free tetris' so if you're looking for mental stimulation and a fun thing to do to kill some time, play Tetris!