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.

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.)

Lumbering Satyr

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

Acid Rain

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.

Fruition

You gain 1 life for each Forest on the battlefield.

Yavimaya, Cradle of Growth

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

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.)

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.

Coiling Woodworm

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

Elvish Champion

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

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.

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.

Llanowar Druid

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

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.

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.

Blanchwood Armor

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

Primal Bellow

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

Living Lands

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

Lifetap

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

Timber Protector

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

Quirion Ranger

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