Spirit and Steam -- A steampunk world which I'm still very fond of. An alternate-history setting, the premise is that magic is real. The Christian church believes magic is therefore fine; it stems from God and is therefore pure and can be good; the church makes it a priority to teach those with the magegift. Science, however, stems from man and is therefore tainted by original sin, and the Anglican church in England in particular regards 'technologists' -- those who study machinery or steam power -- to be heretics. Persecuted religiously throughout Europe, technologists flee to the New World and become the core of the new Colonies. When they rebel and achieve their independence, they defend their coasts effectively by having metal-hulled steamships in their navy, something the Church regards as an abomination. The era is this world's equivalent of the Victorian era, and I tried several variant campaigns set within the world. It's also the world of a number of my short stories (and a stalled novel), which follow a Church-trained mage and a young technologist who both work for the Crown's intelligence service in England, and have had to put aside their differences in the line of duty.
Steamships of the Stars -- A friend described this campaign setting as 'Star Wars written by Jules Verne.' Interstellar travel is real, but in the style that Jules Verne would have envisioned; steam, giant cannons, and things like that. The worlds are tied together in a vast empire ruled by a single man; this emperor is rumored to be immortal due to his possession of a machine which can literally transfer his soul into a new body when he becomes too aged or ill or injured. The players, of course, were to be part of a rebellion against this man from out in the colony worlds. Never started.
Othernight -- Probably my most notable campaign. kieri wanted to try some plot-driven tabletop-style RP, around the time I had just introduced her to Escaflowne. Since chaosrunner and wonderwombat both wanted to play in a tabletop as well, I decided to run it online (as I do most of my 'tabletop' games these days, allowing friends from elsewhere to play). Inspired by diverse sources (El Hazard, Escaflowne, Deverry, my own short stories, etc.) Othernight took place in an alternate world, set on a continent which had been nearly hollowed out like a giant bowl; the mountains along the coast held the sea back, but the interior of the continent was below sea level. Several cultures existed here; the ancient N'thari, the seafaring Rethari pirates-turned-merchant-princes, the nomadic horse-riding H'relith, and the desert kingdom of Telaris. The players fell through the Veil, the boundary between the worlds, and ended up in Telaris as a war of darkness was spreading across the land, and got drawn into it. Othernight died off when chaosrunner's schedule no longer coincided, but had another brief spurt of life as Twilight Falling, a MUSH set in the world.
Aberwyn I -- the first incarnation of my 'Aberwyn' world, a world where everything -- and I mean /everything/ -- had been based around magic. The ruling Council of Mages rather stupidly engaged in actions which, to make a long story short, effectively nullified magic within the realm of Aberwyn. Lightglobes stopped working, mystic transport for trade and mercantile stopped completely, and many of the mages -- the ruling class -- went insane. The campaign was set about ten years later, with the once-great land having been divided up into bandit kingdoms ruled by opportunistic warlords. Pretty generic fantasy world, notable only for the naming system I devised where you could immediately trace a person's lineage, social standing, and position of inheritance within their house by their full name. :)
Aberwyn II -- A second crack at the Aberwyn setting, set centuries later. The bandit kingdoms had fallen or prospered, and gradually formed new lasting nations. These nations established their own power-bases, their own trade, and the tales of the magic-ruled nation of Aberwyn dimmed into ancient history and myth. Until one nation found the old ruined capital city of Aberwyn. Knowing that the other nations would band together to fight any one nation who tried to claim the ancient secrets and knowledge of this ruin, the ruler of this particular nation sent ambassadors to all the others, declaring the ruins neutral territory and proposing a joint excavation and study. Set in the wilderness around these ruins, with a backdrop of exploration, discovery, and political intrigue between the various groups sent by the differing nations, this campaign was planned out but never got started.
I am running one campaign presently, though, called Slipstream. Slipstream combines elements of Aberwyn (the naming system), Othernight (the Rethari reappear in spirit as the Rethinali, simply because I love their culture too much to not use it again in some form), Victorian-era culture, and various other things. I have an older draft of the Slipstream player's handbook PDF I wrote online for, too, for friends who are interested/curious.