One of the first settings for roleplaying games. It began in 1966 when Greg Stafford was trying to pull some wiccan chicks into his wargaming group. Glorantha is based off of themes found in myth rather than from Tolkein's works, and given the fact that Greg is a practicing shaman with a degree in Comparative Religion it's completely batshit.
The "world" is a cube floating in an endless sea topped with a dome of stars that is really the underside of a land of gods and myths. As for other planets there are the various moons which seem to do little more than fuck everything up which is fortunately not often (unless other civilizations or cults get in the way). Everything that occurs in Glorantha generally has some sentient being or magical force responsible for it. For example, all rivers flow in any particular direction because a god had decided, been forced, or been bribed to be there (as the god and the river are one in the same). The people/elements/plants/ducks/lizardmen that inhabit Glorantha are generally religious zealots or ingrates that are more than willing to be extorted by any local gods that they have the poor fortune to witness. Aside from these Neanderthals there are a number of civilizations that seem to get by okay when they pool their efforts and trade off with the stronger gods. That is, except for the Lunar Empire, which seems to fuck everyone's shit up by using a mysterious magic known as logical reasoning. Aside from the Lunars, there is also the Orlanthi who follow Orlanth (very original guys) and fought the Lunar Empire in order to get their own videogame and spotlight status. Besides these major players, Glorantha contains more individual peoples and places than can be covered in a singular paragraph.
Recently a new, gigantic lore book titled "Guide to Glorantha" was released (12 lbs of lore) and covers everything (generally speaking). Sadly it is only available in pdf format and unlikely to be reprinted, but it did make enough money to keep the setting going with new lore.
For more reading pirate the book on /tg/ or support Greg's retirement fund.