So, I've ended up setting up two different Planescape games - one at RPG.net, and one on the the Giant In The Playground forums.
For those of you not in the know, Planescape is a fantastic D&D setting, based around the idea of numerous (somewhere between "many" and "lots") planes of existence, held together by The Great Wheel. Right at its centre is Sigil, the City of Doors - a "hub" where many of these planes overlap, and you can find anything down the right back alley.
Planes can represent the building blocks of creation (Like The Elemental Planes of Fire, Earth, etc. and the Para-Elemental Planes, where they overlap to form things like The Plane of Ice (Air+Water) or Magma (Fire+Earth)), concepts like Life and Death (the Planes of Positive and Negative Energy), or can be afterlifes according to what you believed in in life - if you were all sweetness and light and Boy-Scouted your way through things, you would reach Celestia (fluffy-cloud Heaven) - if you raped and pillaged and kicked puppies every day of your life, it's The Abyss for you!
This is the kind of game that doesn't allow you to pull out old classics like "Bugbears are attacking the next village over!" as plot hooks - Planescape is all about the character's motivations, exploration, spirituality in a place where you can go and see Heaven and Hell (and possibly wander into either while trying to find the loo!), politics amongst the Factions that populate Sigil, interactions amongst all the races of the rainbow you'll find spread across the Planes (the monastic Githzerai, the demon-blooded Tieflings, the Elementally touched Genasi... the list goes on) or, in some spectacular cases, the politics between these races (the millenia-long Blood War between the Lawful Evil Devils, and Chaotic Evil Demons, provides a brilliant in-depth background for a thousand campaigns, or just general fluff for others).
Pretty heavy stuff. But fun, none the less! And of course, sometimes it's still the case that bugbears are attacking the next plane over.