A lot of these have already been recommended, but since I have this handy I'll just re-post my blanket recommendations.
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Here are some story hours I recommend.
(contact)'s
Return to the Temple of Elemental Evil -- note that this is not the original, original thread; rather, it's a reposting of it.
complete
(contact)'s
Liberation of Tenh -- a follow-on to the above story, with some of the same characters. Affectionately known as, "Heydricus kills the sh*t out of them."
incomplete; abandoned
(contact)'s
The Risen Goddess -- less well known that the other two, this is the most philosophical of the three stories. It begins: "Four adventurers sit around a familiar table in a familiar inn, not too far from a place they must have surely known their whole lives-- if they could only remember any of it."
incomplete; abandoned
barsoomcore's
Barsoom story -- very well written. You won't know what the hell is going on half the time, but you won't be able to stop reading despite that.
incomplete; abandoned
Capellan's
Company of the Random Encounter -- don't let the name of the story fool you; this is worth reading.
don't know status
Capellan's
In Hextor's Name -- to paraphrase the author, the story hour is "written from the limited and very, very biased point of view of Kull Redfist, LN Half-Orc Cleric of Hextor, Life's ambition: to become a mighty warrior and crush Hextor's enemies."
complete, I think
Destan's
Sins of our Fathers and
Sins of our Fathers II -- grim, gritty, and utterly compelling, with a definite old school feel to it.
incomplete; abandoned?
James McMurray's
Return to the Tomb of Horrors -- you know about the original ToH, you know about the Return, but you do not know the pleasure of this story hour's approach to it. This is the story hour that made me appreciate high-level play. It's also the first story hour I ever read on ENW.
complete (more or less, you'll understand if you read it)
Rel's
Faded Glory -- proving that pitting the PCs against challenges 4+ ELs above their level makes for a great game. The story takes place in Old One's campaign setting, which is modeled on ancient Rome.
complete
Rune's
Oriental Adventures in the Dream -- the most mind-blowingly original campaign setting I have ever seen... "The world: Ah yes, the world... It is flat, but not really. There is no sun; there are no stars, nor moons. Day and night do exist, however. There is seasonal change (how could we have haiku without it?), but that change is sporadic."
incomplete; abandoned
Sagiro's Story Hour -- epic in scope, with a large cast of characters. It took me a while to get into this story, but once I did, I was hooked. You will be too.
ongoing; sporadically updated
Sepulchrave's
Tales of Wyre, or, Lady Despina's Virtue -- you cannot beat this opening: "One of the PCs, a 14th level Paladin, [...] is currently attempting to CONVERT a succubus, and demonstrate to her the error of her ways."
ongoing, but possibly abandoned
spyscribe's
Welcome to the Halmae -- the aptly named author of this tale keeps it regularly updated, unlike some of those OTHER authors. The campaign starts out small, but grows rapidly into a grand series of quests. Read this story because... justice demands it.
ongoing
Wulf's Story Hour -- follow the adventures of Wulf Ratbane, dwarven bad-ass. Or would that be bad-axe? In any case, one of the strongest archetypal characters of any story hour on the boards.
complete
For a second opinion, check out
this thread or
this one.