Standard Procedure

{1}, Reveal Standard Procedure from your hand: Choose an instant or sorcery card with mana value 3 or less currently legal in the Standard format. Standard Procedure becomes that card until end of turn.

Pietra, Crafter of Clowns

Haste
Robots you control get +1/+1.
{R}{W}, {T}, Tell a joke you haven't told this game to someone outside the game: Create a 1/1 white Clown Robot artifact creature token. If that person laughed, the token gains haste until end of turn.

Underdome

{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to pay Un-costs.

Goblin Tutor

Roll a six-sided die. If you roll a 1, Goblin Tutor has no effect. Otherwise, search your library for the indicated card, reveal it, put it into your hand, then shuffle.
2 — A card named Goblin Tutor
3 — An enchantment
4 — An artifact
5 — A creature
6 — An instant or sorcery

Jack-in-the-Mox

{T}: Roll a six-sided die. This ability has the indicated effect.
1 — Sacrifice Jack-in-the-Mox and you lose 5 life.
2 — Add {W}.
3 — Add {U}.
4 — Add {B}.
5 — Add {R}.
6 — Add {G}.

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

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

The Big Top

{T}: Add one mana of any color that appears on your top. (A top is an outerwear garment worn on the upper body.)

Sacred Foundry

({T}: Add {R} or {W}.)
As Sacred Foundry enters, you may pay 2 life. If you don't, it enters tapped.

Assembled Ensemble

Vigilance
Assembled Ensemble's power is equal to the number of Robots you control.
Whenever you cast a spell with an artifact creature in its art, create a 1/1 white Clown Robot artifact creature token.

Starlight Spectacular

Parade! — At the beginning of combat on your turn, choose creatures you control one at a time until each creature you control has been chosen. Each of those creatures gets +1/+1 until end of turn for each creature chosen before it. (Places everyone! The first creature in line gets +0/+0.)

One-Clown Band

{2}{R}: Target Robot gets +2/+0 until end of turn.

Pie-roclasm

Pie-roclasm deals π damage to each non-Clown creature. (How to calculate π isn't important. It's a smidgen more than 3.)

Omniclown Colossus

Affinity for Clowns (This spell costs {1} less to cast for each Clown you control.)
Trample

Omniclown Colossus // Pie-roclasm

Comet, Stellar Pup

0: Roll a six-sided die.
1 or 2 — [+2], then create two 1/1 green Squirrel creature tokens. They gain haste until end of turn.
3 — [−1], then return a card with mana value 2 or less from your graveyard to your hand.
4 or 5 — Comet, Stellar Pup deals damage equal to the number of loyalty counters on him to a creature or player, then [−2].
6 — [+1], and you may activate Comet, Stellar Pup's loyalty ability two more times this turn.

Arid Mesa

{T}, Pay 1 life, Sacrifice Arid Mesa: Search your library for a Mountain or Plains card, put it onto the battlefield, then shuffle.

Clown Car

When Clown Car enters, roll X six-sided dice. For each odd result, create a 1/1 white Clown Robot artifact creature token. For each even result, put a +1/+1 counter on Clown Car.
Crew 2

Juggletron

Trample
Whenever Juggletron attacks, you may put an art sticker on it.
Juggletron gets +3/+0 for each item in its art that's not touching Juggletron, another item, or the card frame.

Krark's Other Thumb

If you would roll a die, instead roll two of those dice and ignore one of those results.

Clowning Around

Create two 1/1 white Clown Robot artifact creature tokens, then roll a six-sided die. If the result is equal to or less than the number of Robots you control, create a 1/1 white Clown Robot artifact creature token.

Mana Flair

Add {R} for each nonland permanent with art by the artist of your choice.

Battlefield Forge

{T}: Add {C}.
{T}: Add {R} or {W}. Battlefield Forge deals 1 damage to you.

Surprise Party

When Surprise Party enters, yell "Surprise!" and put onto the battlefield any number of printed Clown Robot tokens that aren't touching each other, that you hid on the battlefield before you cast Surprise Party, and that weren't spotted by an opponent before you cast Surprise Party.
Robots you control get +1/+0 and have vigilance.

Yet Another Aether Vortex

All creatures have haste.
Players play with the top card of their libraries revealed.
Noninstant, nonsorcery cards on top of a library are on the battlefield under their owner's control in addition to being in that library.