Merrow Reejerey

Other Merfolk creatures you control get +1/+1.
Whenever you cast a Merfolk spell, you may tap or untap target permanent.

Cephalid Coliseum

{T}: Add {U}. Cephalid Coliseum deals 1 damage to you.
Threshold — {U}, {T}, Sacrifice Cephalid Coliseum: Target player draws three cards, then discards three cards. Activate only if seven or more cards are in your graveyard.

Mistform Ultimus

Mistform Ultimus is every creature type (even if this card isn't on the battlefield).

Winter Orb

As long as Winter Orb is untapped, players can't untap more than one land during their untap steps.

Zephyr Spirit

When Zephyr Spirit blocks, return it to its owner's hand.

Cephalid Snitch

Sacrifice Cephalid Snitch: Target creature loses protection from black until end of turn.

Kjeldoran Outpost

If Kjeldoran Outpost would enter the battlefield, sacrifice a Plains instead. If you do, put Kjeldoran Outpost onto the battlefield. If you don't, put it into its owner's graveyard.
{T}: Add {W}.
{1}{W}, {T}: Create a 1/1 white Soldier creature token.

Stasis

Players skip their untap steps.
At the beginning of your upkeep, sacrifice Stasis unless you pay {U}.

Army of Allah

Attacking creatures get +2/+0 until end of turn.

Replenish

Return all enchantment cards from your graveyard to the battlefield. (Auras with nothing to enchant remain in your graveyard.)

Serra's Sanctum

{T}: Add {W} for each enchantment you control.

Common Cause

Nonartifact creatures get +2/+2 as long as they all share a color.

Leviathan

Trample
Leviathan enters the battlefield tapped and doesn't untap during your untap step.
At the beginning of your upkeep, you may sacrifice two Islands. If you do, untap Leviathan.
Leviathan can't attack unless you sacrifice two Islands. (This cost is paid as attackers are declared.)

The First Doctor

When The First Doctor enters the battlefield, search your library and/or graveyard for a card named TARDIS, reveal it, and put it into your hand. If you search your library this way, shuffle.
Whenever you cast a spell with cascade, put a +1/+1 counter on target artifact or creature.