Sudoku
Nine by nine, every digit once in each row, column and box.
A grid part-filled with digits and one rule: each row, each column and each of the nine boxes holds the digits one to nine exactly once. Nothing is guessed — every square that can be filled can be reasoned out from the squares already filled, and a puzzle worth solving has exactly one answer.
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.