MMOs (massively multiplayer online games) that do not require a monthly subscription fee to play or the purchase of a client are called free-to-play (F2P). These free MMOs generate revenue through the use of micro-transactions, small purchases which let the player get special items, bonuses, or other perks to use within the game.
While free-to-play MMOs were originally popular in the East, they have become increasingly popular in the West. Popular and well-known free MMOs include Runes of Magic, Dungeons and Dragons Online, Lord of the Rings Online, and MapleStory, and many popular pay-to-play MMOs, such as World of Warcraft and Warhammer Online, are predicted to become free-to-play in the future.