For a long time - years maybe - I've had this world in my mind (you have worlds in your MIND?! O.o) called Drisque, where it is eternally raining. Different regions receive different amounts of rain, and there are seasonal weather patterns and stuff, but it's mostly constant - this is a planet which has not changed in millions of years, maybe. It has several races, probably more than I've yet thought of: the naiads, who live in rivers but are capable of walking on land; the mer, maybe, but I'm not really sure as the ocean is not an important part of it yet; the humans, who are probably travellers from another world, as they are certainly not native to Drisque (this is fantasy, not sci-fi, but it has somewhat unique magic); the Kgelthri, or frog-folk, who have a vast, civilized empire; the Ahrelthri, who are the very mysterious mushroom folk, given their name by the Kgelthri, having no oral language of their own - they communicate with pheromones, very low frequency vibrations, flickers of multicolored light, and maybe telepathy, and they are not mushroom men, they are entire, mostly stationary trees. There are also the swamp dragons, maybe, but I'm not certain yet, and the forest elves in the jungles, maybe, but again, I'm not certain yet. Beyond that I don't really know about the races.
As for the regions, there are many, and I keep meaning to draw a digital map and never actually do: the Savraen Moor, which receives light rain mostly, just a sprinkle, but enough that you need an umbrella, sort of like a vast, expansive grassland with few trees but lots of grasses (purple, blue, and cyan - the green plants are farther to the south, in the jungles) that soak up what little sunlight gets through the overcast atmosphere - perhaps there's some other source of energy I don't know about though, as without direct sunlight there is really no hope of plantlife, but it is a lush environment, in general. Through the Savraen Moor runs a few rivers (I forget their names, I have it written down somewhere), home to naiads, coming from a mountain range whose name I also cannot remember, dang it. South of the Moor (or Savrae, as it is called for short) is the Mirrowmarsh, a mildly swampy region dominated by vast mushroom trees and phosphorescent plants, in many colors - a very beautiful place, but where one has the sense that one is being watched, mainly because one is - the mushrooms are aware, intelligent, they are the Ahres, but their name for themselves, if there is one, is unknown. There is an uncanny, dejavu feeling throughout, but they are mostly benevolent, or at least neutral. Rarely do they communicate with outsiders, but it has been known to happen. Those who threaten them, however, are never seen again.
To the south of the Marsh is Kalendi ar-Drossa, the translation of whose name I also can't remember (bother, I'll have to go back and look at my notes from a long time ago later - I think it means Kingdom of Splendor or something), the empire of the Kgelthri, with their capital at the fine city of Chemma. They actually live amongst the ahres, having an ancient alliance with them - hence the term "toadstool." The Kgelthri are somewhat vulgar in their manners, but they have awesome opera singers, and they are generally quite honorable. To the east of the Kingdom is the Chasm of Cairenn, a mysterious and creepy chasm in the earth which has been growing in recent years, slowly but surely - a cursed place. Rumor has it that there have been sightings of mysterious, warped, twisted creatures nearby (I'm a fan of Lovecraft). Most don't go near it. To the east of that is the dark, creepy Dragonwood Fens, home to the swamp dragons, an evil and twisted race (a disgrace to dragons in general, who are usually quite nice) - rumor has it that they have an alliance with the chasm creatures and are getting ready to invade the rest of Drisque. To the north, past another range of mountains, secluded from the rest of Drisque by these mountain ranges, is the Bog of Giranden, home to the humans - think of Louisiana here, it's similar in culture and style, lots of folk ritual, voodoo, bayous and cypresses, all the best stuff, and it is ruled by the ancient, mysterious Swamp Witch, Giranden, of whom little is known except her extreme power. She's a nice lady, very grandmotherly - but you can tell that ancient secrets lurk behind her golden brown eyes.
There's other places, but that description should do for now, eh?