Euchre, 500: As the highest trump or "top Bower".
Canasta: The joker, like the deuce, is a wild card. However, the joker is worth 50 points in melding, as opposed to 20 for the deuce.
Gin Rummy: a wild card, able to be used as any necessary rank or suit to complete a meld.
Chase the Joker: An alternative version of Old Maid where the Joker card is used instead of the Ace.
War: In some variations, beats all other cards.
Pitch: A point card in some variations. Jokers usually are marked as "High" and "Low", one outranking the other.
Mighty: Second most-powerful card in the game, though it cannot be legally played on the first or last trick.
Daihinmin: a wild card, or a deuce (which ends the round and clears the discard pile).
Crazy Eights: a "skip" card, playable on top of any other card, that forces the next player to lose a turn.
Spades: uncommon, but can fulfill one of two roles. When playing with three or six players they are added to make the cards deal evenly (18 or 9 cards each, respectively). They are either "junk" cards playable anytime that cannot win a trick, or they count as the two highest trumps (the two Jokers must be differentiable; the "big Joker" outranks the "little Joker"). They also can be used in conjunction with teammates cards to create a pseudo-"trump", i.e. an Ace of Hearts and Joker played together would be counted as an Ace of Spades, inferior only to a natural Ace of Spades.
Double King Pede: As the lowest-ranked card, but worth 18 points.
Go Fish: In a game with two players, the joker pair is often used to bring the number of pairs to 27 and prevent a 13-13 tie.