Minesweeper
Every number tells you how many mines it is touching.
A field with mines hidden under it. Open a square and it either ends the game or shows a number: how many of its eight neighbours are mined. Flag the ones you have worked out, open the ones you are sure of, and clear the field. The first click is always safe; after that it is arithmetic, until the last two squares, which occasionally are not.
In development
The game is not here yet — this page is its address, waiting for it. Nothing to install and nothing to sign up for when it arrives.