Pentaballs HD is a very nice variation of tic-tac-toe. Instead of attempting to get three crosses or circles in a row in a three-by-three board, in Pentaballs, you play on four three-by-three boards that you can rotate at will.
The objective is similar; you have to line up five marbles in a row, either horizontally, vertically or diagonally. In each turn, you place one marble on a slot of any of the blocks and you also get to rotate one of them. Your opponent does the same turn after turn. Whoever gets five in a row wins. This is way harder than it sounds, though. Not only can your opponent put marbles in your way, but they can also stop your attempts by rotating the block where your marbles are, or the one next to it. You need at least two blocks to get five marbles in a row, so it is important to calculate your moves.
You can play Pentaballs against a computer or a human opponent. When you play against a friend, both of you take turns on the same computer. There is no online multiplayer mode.
In short, Pentaballs is quite an interesting game to play to kill some time. It is more complicated than tic-tac-toe but fun nonetheless.