One of the most important points of strategy in Magic: The Gathering is learning to maximize your mana production. Green is by far the best color for mana-producing creatures. There are many small creatures which cost one or two mana to cast and have multiple uses, including tapping to produce mana. Using these Green mana-producing creatures can help you produce enough mana to get larger creatures out faster, giving you an advantage over your opponents.
Basic Mana Production: Tap for One Green Mana
The basic Green mana-producing creature is Llanowar Elves. Its casting cost is one Green mana and it is a 1/1 Elf Druid. Llanowar Elves can tap to produce one Green mana.
Many other 1/1 Green Elf and Druid creatures have the same ability. These include Fyndhorn Elves. There are also some creatures which can tap to produce one Green mana but have a higher power and/or toughness, and consequently cost more to cast. These include Leaf Gilder and Wirewood Elf.
Tap for Multiple Green Mana
Some Green creatures can tap to produce multiple Green mana. Rofellos, Llanowar Emissary taps to produce one Green mana for each Forest you control. At a casting cost of just two Green mana, this is a very powerful card. Another is Fyndhorn Elder, which costs one Green mana and two colorless mana and can tap to produce two Green mana.
Tap to Produce Multiple Colors of Mana
In a multicolor deck, Green creatures that can tap to produce multiple colors of mana are especially useful. These creatures give you the opportunity to cast certain spells even if you hit a mana screw and can't get the proper colors of land out fast enough. The basic example is Birds of Paradise, a 0/1 flying creature with a casting cost of one Green mana. Birds of Paradise can tap to produce any color of mana, a very powerful ability. Utopia Tree, a 0/2 Plant creature, has the same ability.
Other Green creatures can tap to produce certain limited colors of mana, or allow you to pay a colorless mana in exchange for particular color of mana. Noble Hierarch, a 0/1 Human Druid, can tap to produce one Green, White, or Blue mana. Skyshroud Elf can tap to produce one Green mana or allow you to pay one colorless mana to produce one White or Red mana.
Cause Other Creatures to Tap for Mana
Another useful ability is giving all creatures of a certain type the ability to tap to produce mana. Seton, Krosan Protector, for example, is a Centaur Druid Legend which allows you to tap other Druids to produce one Green mana. Heritage Druid allows you to tap three untapped Elves to produce three Green mana.
Advantages of Mana-Producing Creatures
There are many more Green mana-producing creatures. Keeping many at hand will give you an advantage, especially in a multicolored deck, because they have a low casting cost and, unlike land, you can play more than one per turn. An army of 1/1 or 2/2 Elves and Druids can attack and block as well as produce mana. Mana production is Green's strength, and these creatures are an essential part of a Green or multicolored deck.