Reversi — free browser game

Play as Black on an 8×8 board against the computer's White; the side with the most stones at the end wins. Click or tap a cell to place — legal moves are highlighted in advance; R resets the game. A move is legal only if it sandwiches at least one of the opponent's stones between the new stone and an existing stone of your colour along a horizontal, vertical, or diagonal line. Every sandwiched white stone flips to black at once and a single move can trigger flips in multiple directions. With no legal move you pass; two consecutive passes (or a full board) ends the game and the bigger pile wins. Tip: corners can never be flipped so they're gold — but the squares adjacent to corners (the X and C points) give the corner away cheaply, so don't open them early. In the opening, flipping fewer stones to restrict the opponent's move options is usually stronger than greedily flipping many.