Jack Frost, scowling star of the final game in Nitrome's informal 2007 winter trilogy (after Thin Ice and Snow Drift), is happiest when he is cold. Your job is to turn 40 levels of autumn-colored blocks and ladders into an arctic wonderland. Control Jack with the arrow keys: [up] to jump, [left] and [right] to walk, [up] and [down] to climb. Simply step on blocks or climb ladders to freeze them. Q*Bert will try to stop you, along with about a dozen other types of critter, but you can fight most of them by bouncing on their heads to temporarily encase them in a solid cube of ice.
Park a friend on the WASD keys, and the two of you can spread wintry devastation simultaneously. The 2-player mode is staged on the same set of levels as 1-player is, and you both work towards the same goal, but there is a competition aspect to it. At the end of a level, whoever has turned more blocks to his/her own color wins the round, and the game keeps track of who is in the lead as you progress. Co-op play is rare in the video game world, and even rarer in free online games, so Nitrome gets major points for including this option.