DONUTS, which was first playable as INSERT, is an abstract strategy game for two players in which you attempt align five rings of your color to win.
To set up, arrange the four 3x3 tiles at random into a 6x6 grid. Each square of the grid has a vertical, horizontal, or diagonal line on it. The first player places one of their rings on any unoccupied space, and the line in that space indicates the direction in which the opponent must place their ring: vertically, horizontally, or diagonally in line with the ring just placed. If the opponent can't place a ring in this direction because each square in this line is occupied, then the opponent can place a ring in any unoccupied square.
If you manage to insert a ring between two rings owned by the opponent — whether by placing your X piece in a space like this O_O or in a space like this OXX_O — then you change those opposing rings to your color.
If the board is full and no one has managed to create a row of five rings in their color, then the player with the largest orthogonally connected group of rings wins.