MAGIC: THE GATHERING
Until end of turn, whenever a player taps an Island for mana, that player adds to his or her mana pool (in addition to the mana the land produces).
High Tide is such a strong combo enabler that there is even a Legacy deck named after it. When combined with the ability to untap lands (Turnabout, Candelabra of Tawnos, Time Spiral, etc.), High Tide can create obscene amounts of storm, copious amounts of mana, and a lot of mill potential.