Reversi

Reversi is played on an 8x8 board. You place a disc so that it brackets one or more of your opponent's discs between your new disc and a disc you already own - every bracketed disc flips to your colour. Black moves first. If you cannot make a legal move, you pass. When neither player can move, the player with the most discs on the board wins.

Almost, but not quite. Reversi is the public-domain game; Othello is a trademarked variant with two rule tweaks - in Othello both players start with the same four-disc cross, and the game ends when the board is full. For casual play the names are used interchangeably, and this version follows the standard Reversi rules.

Corners are everything, because a captured corner can never be flipped back. Beyond that: avoid the C-squares and X-squares adjacent to an empty corner (they hand the corner to your opponent), and prioritise stable discs - discs on an edge or in a completed row that can no longer be flipped. Counting discs early is a trap; mobility, meaning how many moves you have available, matters far more.

Yes. This version includes a computer opponent, so you can play a full game solo with no sign-up. You can also pass the device back and forth to play against another person on the same board.

Yes - completely free, with no download, no installation and no account. It runs directly in your browser on desktop and mobile, and it keeps working offline once the page has loaded.