Crossing the addictiveness of Tetris with the basic math of Sudoku, Drop7 was a big viral hit on the Web before it moved to the Apple iPhone. The setup is simple: Numbered circles appear at the top of the screen, and you have to drop them into columns. Line them up in the quantity written on the circle – for example, two twos in a row, or three threes, and so on – and the circles explode, sometimes setting off chain reactions as new combinations slide together. Chain reactions boost your score exponentially, but you will also deal with circles whose numbers are hidden in shells that must be steadily broken apart, mixing dumb luck into your best-laid plans.

Unlike Tetris, your session can’t go on forever; the concealed numbers will eventually pile up beyond your ability to crack them open, and when the numbers exceed the columns that hold them, you are done for. But the game’s abacus-like aesthetic compels you keep giving it one more try. Superior players can line up astounding chain reaction scores, but a casual player can have just as much fun watching the numbers vanish row by row.

But the most surprising thing about Drop7 is its background story. The game was created by the developer Area/Code as a front for an alternate-reality game named Chain Factor (chainfactor.com), built to support the television show Numb3rs. Serious players explored an intricate storyline through fake Web sites and clues on public billboards. Meanwhile, casual fans could just swing by to play Drop7 – and their number crunching would still aid the experience by unlocking more content for the hardcore crowd. This gave players a sense of mystery, as they helped a cause they didn’t understand and aided a community that was larger than themselves – all by simply sliding numbers down a screen.