The three I have DMed, in order from the most used to the least used:
Mäelström
The world I use for most of the adventures. It is a very classic D&D setting.
- geography: it has its own geography but with many locations borrowed from Forgotten Realms and others from some friends' homebrew material; it is not really planned beforehand, but it rather developes in time, every adventure opening to a new area. The most striking feature is that it is actually a planet which almost "rolls" on its orbit (like Uranus, in case someone knows...), which has the consequence of having day/night and seasons only near the equator, while the north is a desert of neverending daylight and summer, and the south is a frozen ocean of eternal darkness
- ecology: contrary to more typical settings, there aren't thousands of different creatures, only about 20-30% of the MM exists (up to now), but I tried to make them important somehow. Humans are the vast majority, demihumans are quite secluded in their own areas, Orcs form a tribal but very solid society (ruthless and with evil tendencies but individuals can be anything), Goblinoids don't have a society but instead mingle with everyone, half-races are more rare than subraces and definitely scattered; some others have made an important appearance with their own society but very limited in number (many giants, centaurs, drow, trolls, rakshasa); all aberrations and ooze are rare and unique creatures created artificially just like constructs (and undead, but they are quite common)
- politics: not very original, more or less there are background rivalries and alliances between feuds and the same happens between Orc tribes; in general there is hostility between humans and Orcs, but not trivial. Other large parties are the druidic society (scattered and ambiguous) and the outsiders (obviously separated between Devils, Demons and Celestials - nothing else yet) who have periodic plans over the world. Several organizations from few individuals to hundreds play a much more relevant role to player's adventures
- religion: in theory I should have developed a well-structured pantheon, but then I ended up allowing players to worship or be cleric of any Greyhawk or FR deities (and even Tyche in one case). It is not clear to the characters how much their divine powers really comes from the deity and how much comes from simply their own faith, but deities often dictates events on earth (this uncertainty has definitely inspired the name I gave to the setting)
- cosmology: the Great Wheel from MotP but 8 of the outer planes are instead alternate material planes, with many variations compared to MotP; anyway I didn't have occasion yet to run a full adventure there, but many are ready to be explored
Nadir
It is a world where evil has won, and gods and celestials have lost their interests and abadoned it...
- geography: basically a single huge continent surrounded by oceans; wildlife has mostly disappeared due to heavy exploitation of the environment, and the sky is always cloudy and darklooking; not much developed yet besides the few starting locations, but many ideas already brewing
- ecology: mostly the continent is dull and repetitive with people living concentrated in a few cities; potentially everything can be met outside those cities, while all races live within in a mixed society
- politics: the idea is of a setting where evil is the normal thing, with lots of challenges for RPing and DMing (imagine you were born in a country where many evil acts are praised while some good acts are forbidden...). This means that about half people is evil and the other half is neutral (but accustomed to evil being the norm), while good people are rare and tend to become PCs
. Nevertheless many factions are fervent and this is suppsed to provide lots of intrigue, tyranny, betrayal, scheming etc.
- religion: as I said, deities have abandoned the world and there are no churches; Mystics have either learned what the world used to be before or "feel" how it could become in the future and gain some divine spellcasting, while Earthchilds have a similar connection to what's left of nature
- cosmology: besides transitive planes there is basically nothing else, or rather it doesn't matter what there is because the access is cut for mortals
Note: this setting is something I tried only a very few times and at that moment I used some custom classes instead of the PHB (Commander, Swashbuckler, Warrior, Earthchild, Mystics, Healer, Clairvoyant, Mage, Summoner, Necromancer). It was early times of gaming and they were very bad designed
Boadicea (I know, it's a stolen name...)
Very much underdeveloped and tried only for a few evenings, this is a Celtic setting with mostly fairy races (good and evil) and with only 3 classes: Fighter, Rogue and Druid, but somehow customizable - e.g. barbarian's abilities as feats. If we had played enough I would have allowed Bards, Rangers and Paladins (of a deity, any alignment) as PrCls.
- geography: not developed besides the adventure locations, probably very much earthlike
- ecology: MM only with few extras, in any case creatures are selected based on the flavor (e.g. yes to ghosts and bashees, no to zombies and mummies) and are supposed to be always rare, as only PCs really have "adventures"
- politics: for the moment pretty non-important for the PCs
- religion: very many religions (both historical and fantastic) but the approach is like in real life - religion is a matter of belief and doesn't grant powers (given time this might have changed if I allowed Paladins). There are priests but they could be any class and don't have their own. Druids are essentially mages and since they are the only spellcasters there is no distinction between arcane and divine
- cosmology: not developed, but probably at least a heaven and a hell would have been used early or late
