Exit Through the Grift Shop

Yell "Who wants a souvenir?" and announce the name of a nonland card in your graveyard. Each player in an Un-game who can hear you may bid life. You start the bidding at 0. The high bidder loses life equal to the high bid. They copy the card and may cast the copy without paying its mana cost. If you didn't win the bid, you gain life equal to the high bid and create that many Treasure tokens.

T.A.P.P.E.R.

{T}, Tap an untapped creature you control that has a hat: Tap target creature.

Sly Spy

Whenever Sly Spy deals combat damage to a player, you may destroy target creature facing left in its art. (Creatures without faces don't face anywhere.)

Now You See Me . . .

Exile target creature you control, then return it to the battlefield under its owner's control with X +1/+1 counters on it, where X is the number of mirrors on walls you can see from your seat.

"Brims" Barone, Midway Mobster

When "Brims" Barone, Midway Mobster enters, put a +1/+1 counter on each other creature you control that has a hat.
"Brims" Barone, Midway Mobster has menace as long as you're wearing a hat.

Five Kids in a Trenchcoat

Five Kids in a Trenchcoat counts as five creatures for spells and effects that count the number of creatures you control.

Form of the Approach of the Second Sun

When this enchantment enters, you gain 7 life. You become a card until you leave your library or that library is shuffled. Put yourself seventh from the top, balancing the cards on top of you on your head.
When you draw yourself, you win the game.
When one or more cards fall off your head, exile them and all cards on your head, then sacrifice this enchantment.

Hat Trick

Target blocking or blocked creature you control gets +1/+1 and gains first strike until end of turn. If it has a hat, it gains double strike until end of turn. If you and that creature each have a hat, the creature gains triple strike until end of turn.

Some Disassembly Required

{B}, Sacrifice a creature: Distribute the sacrificed creature's keyword abilities among any number of other target creatures until end of turn. If it's December, you gain 1 life for each of those keywords.

Knight of the Kitchen Sink

First strike, protection from loose lips (Nothing with an open mouth in its artwork can block, target, deal damage to, or attach to this creature.)

Knight of the Hokey Pokey

First strike
{1}{W}, Do the Hokey Pokey: Prevent all damage a source of your choice would deal to Knight of the Hokey Pokey this turn. (To do the Hokey Pokey, choose an arm, a leg, or your whole self and put it in. Put it out. Put it in. Shake it all about. You do the Hokey Pokey! Turn yourself around.)

Syr Cadian, Knight Owl

Knightlifelink (Damage dealt by Knights you control also causes you to gain that much life.)
{W}: Syr Cadian, Knight Owl gains vigilance until end of turn. Activate only from sunrise to sunset.
{B}: Syr Cadian, Knight Owl gains flying until end of turn. Activate only from sunset to sunrise.

Whenever Sly Spy deals combat damage to a player, you may destroy target creature facing right in its art. (Creatures without faces don't face anywhere.)

Scooch

Add or subtract 1 from target creature's power, target player's life total, or target die roll's result.
Draw a card.

Rat in the Hat

{T}, Sacrifice Rat in the Hat: Until end of turn, you may cast target creature card that has a hat from your graveyard.

Haberthrasher

Deathtouch
Whenever another creature you control that has a hat enters or a hat sticker is put on a creature you control, each opponent loses 1 life.

Park Re-Entry

Return up to two target creature cards that each have a hat and/or mana value 3 or less from your graveyard to the battlefield.

Konda's Hatamoto

Bushido 1 (Whenever this creature blocks or becomes blocked, it gets +1/+1 until end of turn.)
As long as you control a legendary Samurai, Konda's Hatamoto gets +1/+2 and has vigilance. (Attacking doesn't cause this creature to tap.)