Rakdos Signet

Grafted Exoskeleton

Equipped creature gets +2/+2 and has infect. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Whenever Grafted Exoskeleton becomes unattached from a permanent, sacrifice that permanent.
Equip {2}

Sol Ring

Phyresis

Enchant creature
Enchanted creature has infect. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Dimir Signet

Ulvenwald Tracker

{1}{G}, {T}: Target creature you control fights another target creature.

Golgari Signet

Orzhov Signet

Gruul Signet

Phyrexian Altar

Sacrifice a creature: Add one mana of any color.

Maskwood Nexus

Creatures you control are every creature type. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.
{3}, {T}: Create a 2/2 blue Shapeshifter creature token with changeling. (It is every creature type.)

Atogatog

Sacrifice an Atog creature: Atogatog gets +X/+X until end of turn, where X is the sacrificed creature's power.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Megatog

Sacrifice an artifact: Megatog gets +3/+3 and gains trample until end of turn.

Azorius Signet

Karmic Guide

Flying, protection from black
Echo {3}{W}{W} (At the beginning of your upkeep, if this came under your control since the beginning of your last upkeep, sacrifice it unless you pay its echo cost.)
When Karmic Guide enters, return target creature card from your graveyard to the battlefield.

Izzet Signet

Fellwar Stone

{T}: Add one mana of any color that a land an opponent controls could produce.

Boros Signet

Reveillark

Flying
When Reveillark leaves the battlefield, return up to two target creature cards with power 2 or less from your graveyard to the battlefield.
Evoke {5}{W} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)