Lifetap

Whenever a Forest an opponent controls becomes tapped, you gain 1 life.

Elvish Champion

Other Elf creatures get +1/+1 and have forestwalk. (They can't be blocked as long as defending player controls a Forest.)

Timber Protector

Other Treefolk creatures you control get +1/+1.
Other Treefolk and Forests you control have indestructible.

Life and Limb

All Forests and all Saprolings are 1/1 green Saproling creatures and Forest lands in addition to their other types. (They're affected by summoning sickness.)

Primal Bellow

Target creature gets +1/+1 until end of turn for each Forest you control.

Eladamri, Lord of Leaves

Other Elf creatures have forestwalk. (They can't be blocked as long as defending player controls a Forest.)
Other Elves have shroud. (They can't be the targets of spells or abilities.)

Coiling Woodworm

Coiling Woodworm's power is equal to the number of Forests on the battlefield.

Yavimaya, Cradle of Growth

Each land is a Forest in addition to its other land types.

Living Lands

All Forests are 1/1 creatures that are still lands.

Tourach's Chant

At the beginning of your upkeep, sacrifice Tourach's Chant unless you pay {B}.
Whenever a player puts a Forest onto the battlefield, Tourach's Chant deals 3 damage to that player unless they put a -1/-1 counter on a creature they control.

Acid Rain

Fruition

You gain 1 life for each Forest on the battlefield.

Quirion Ranger

Return a Forest you control to its owner's hand: Untap target creature. Activate only once each turn.

Beacon of Creation

Create a 1/1 green Insect creature token for each Forest you control. Shuffle Beacon of Creation into its owner's library.

Lumbering Satyr

All creatures have forestwalk. (They can't be blocked as long as defending player controls a Forest.)

Ambush Commander

Forests you control are 1/1 green Elf creatures that are still lands.
{1}{G}, Sacrifice an Elf: Target creature gets +3/+3 until end of turn.

Baru, Fist of Krosa

Whenever a Forest enters, green creatures you control get +1/+1 and gain trample until end of turn.
Grandeur — Discard another card named Baru, Fist of Krosa: Create an X/X green Wurm creature token, where X is the number of lands you control.

Dungrove Elder

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Dungrove Elder's power and toughness are each equal to the number of Forests you control.

Blanchwood Armor

Enchant creature
Enchanted creature gets +1/+1 for each Forest you control.

Scryb Ranger

Flash
Flying, protection from blue
Return a Forest you control to its owner's hand: Untap target creature. Activate only once each turn.

Llanowar Druid

{T}, Sacrifice Llanowar Druid: Untap all Forests.

Nissa, Who Shakes the World

Whenever you tap a Forest for mana, add an additional {G}.
+1: Put three +1/+1 counters on up to one target noncreature land you control. Untap it. It becomes a 0/0 Elemental creature with vigilance and haste that's still a land.
−8: You get an emblem with "Lands you control have indestructible." Search your library for any number of Forest cards, put them onto the battlefield tapped, then shuffle.