Essence Warden

Whenever another creature enters, you gain 1 life.

Forest

Plains

Herd Baloth

Whenever one or more +1/+1 counters are put on this creature, you may create a 4/4 green Beast creature token.

Trelasarra, Moon Dancer

Whenever you gain life, put a +1/+1 counter on Trelasarra and scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Marauding Blight-Priest

Whenever you gain life, each opponent loses 1 life.

Swamp

Soul Warden

Cleric Class

(Gain the next level as a sorcery to add its ability.)
If you would gain life, you gain that much life plus 1 instead.
{3}{W}: Level 2
Whenever you gain life, put a +1/+1 counter on target creature you control.
{4}{W}: Level 3
When this Class becomes level 3, return target creature card from your graveyard to the battlefield. You gain life equal to that creature's toughness.

Dina, Soul Steeper

Whenever you gain life, each opponent loses 1 life.
{1}, Sacrifice another creature: Dina gets +X/+0 until end of turn, where X is the sacrificed creature's power.

Heliod, Sun-Crowned

Indestructible
As long as your devotion to white is less than five, Heliod isn't a creature.
Whenever you gain life, put a +1/+1 counter on target creature or enchantment you control.
{1}{W}: Another target creature gains lifelink until end of turn.

Scurry Oak

Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)
Whenever one or more +1/+1 counters are put on this creature, you may create a 1/1 green Squirrel creature token.