Theros: Nykthos, Shrine to Nyx
: Add to your mana pool.
, : Choose a color. Add to your mana pool an amount 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.)
Nykthos is an extremely powerful mana generator if you have a mono-color deck. All you need are a bunch of mana symbols -- check out the Spirit Avatars in Shadowmoor for inspiration!