Doubling Cube

{3}, {T}: Double the amount of each type of unspent mana you have.

Omnath, Locus of Mana

You don't lose unspent green mana as steps and phases end.
Omnath, Locus of Mana gets +1/+1 for each unspent green mana you have.

Vernal Bloom

Whenever a Forest is tapped for mana, its controller adds an additional {G}.

Birds of Paradise

Llanowar Elves

Elvish Mystic

Heartbeat of Spring

Whenever a player taps a land for mana, that player adds one mana of any type that land produced.

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.