Khalni Hydra

This spell costs {G} less to cast for each green creature you control.
Trample

Yavimaya, Cradle of Growth

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

Fyndhorn Elves

Utopia Sprawl

Enchant Forest
As Utopia Sprawl enters, choose a color.
Whenever enchanted Forest is tapped for mana, its controller adds an additional one mana of the chosen color.

Wild Growth

Enchant land
Whenever enchanted land is tapped for mana, its controller adds an additional {G}.

Primalcrux

Trample
Chroma — Primalcrux's power and toughness are each equal to the number of green mana symbols in the mana costs of permanents you control.