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.

Myr Superion

Spend only mana produced by creatures to cast this spell.

Forest

Harmonize

Mark of Sakiko

Enchant creature
Enchanted creature has "Whenever this creature deals combat damage to a player, add that much {G}. Until end of turn, you don't lose this mana as steps and phases end."

Llanowar Elves

Emrakul, the Aeons Torn

This spell can't be countered.
When you cast this spell, take an extra turn after this one.
Flying, protection from spells that are one or more colors, annihilator 6
When Emrakul, the Aeons Torn is put into a graveyard from anywhere, its owner shuffles their graveyard into their library.

Doubling Cube

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

Upwelling

Players don't lose unspent mana as steps and phases end.

Arbor Elf

Nykthos, Shrine to Nyx

{T}: Add {C}.
{2}, {T}: Choose a color. Add an amount of mana of that color equal to your devotion to that color. (Your devotion to a color is the number of mana symbols of that color in the mana costs of permanents you control.)

Elvish Mystic

Manamorphose

Add two mana in any combination of colors.
Draw a card.

Magus of the Vineyard

At the beginning of each player's first main phase, that player adds {G}{G}.

Vernal Bloom

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