Naoki Inaba's Marupeke, from maru 丸 (circle) and peke ペケ (X-mark), is a binary puzzle in the family of Takuzu.
The objective is to fill every blank cell in the grid with an O or X so that neither symbol appears three times in a row in any grid row, column, or diagonal.
Click or tap a cell to cycle it from O to X to blank; right-click cycles the other way. Keyboard controls work too: arrows move, O and X mark the cell, Space cycles as clicking does, Backspace erases, and Ctrl + Z undoes your last move.
Puzzles of this sort can require guesswork in the sense that you need to think some steps ahead about the consequences of marking O or X in a cell before you can determine which mark is correct. The difficulty levels determine how often you need to think ahead, with Easy puzzles requiring no planning at all. The puzzle you see above is encoded in the address bar, so you can share a link to this particular puzzle if you like. You can also generate a printable PDF of the puzzle: