Swamp

Wirewood Pride

Target creature gets +X/+X until end of turn, where X is the number of Elves on the battlefield.

Elvish Vanguard

Whenever another Elf enters, put a +1/+1 counter on this creature.

Elvish Herder

{G}: Target creature gains trample until end of turn.

Forest

Wellwisher

{T}: You gain 1 life for each Elf on the battlefield.

Priest of Titania

{T}: Add {G} for each Elf on the battlefield.

Timberwatch Elf

{T}: Target creature gets +X/+X until end of turn, where X is the number of Elves on the battlefield.

Llanowar Visionary

When this creature enters, draw a card.
{T}: Add {G}.

Eyeblight Massacre

Non-Elf creatures get -2/-2 until end of turn.

Wirewood Herald

When this creature dies, you may search your library for an Elf card, reveal that card, put it into your hand, then shuffle.

Elvish Visionary

Abomination of Llanowar

Vigilance; menace (This creature can't be blocked except by two or more creatures.)
Abomination of Llanowar's power and toughness are each equal to the number of Elves you control plus the number of Elf cards in your graveyard.

Lys Alana Huntmaster

Whenever you cast an Elf spell, you may create a 1/1 green Elf Warrior creature token.

Elvish Guidance

Enchant land
Whenever enchanted land is tapped for mana, its controller adds an additional {G} for each Elf on the battlefield.

Vorrac Battlehorns

Equipped creature has trample and can't be blocked by more than one creature.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery. This card enters unattached and stays on the battlefield if the creature leaves.)