I've run several fun one-shots over the last couple of years:
Orcz! - I ran this at NC Game Day II - The party consists of a group of orcs who need to rescue their chieftain (whom a few of them are related to) from a tribe of goblins that captured him. If they don't manage to rescue him, their heads are likely to be on the chopping block because the new chieftain had a grudge against their family. The goblin was in cahoots with a powerful shaman who had learned to chop the heads off of orcs and then enlarge and reanimate the heads. Plus he also reanimated the headless corpses.
Highlights: Lots of slugfest combat, squeaky obsequious goblins, headless orc-zombies and disembodied giant orc heads. This adventure also featured two of the coolest magic items I've ever invented: Daern's Instant Outhouse and the Club of Healing (deals 1d6 damage and heals 1d8+1 HP to anybody struck with it - the cleric in the party beat other party members back to health several times during the adventure).
Return to the Dungeons of the Slave Lords - I ran this at NC Game Day I - A reprise loosely based on the AD&D module A4 - In the Dungeons of the Slave Lords. Essentially the party starts almost naked and must escape from a volcanic island before it explodes.
Highlights: Fun to watch how inventive the party can be when they each start with a canvas tunic, 5' of rope and almost nothing else. You can read a full account of the adventure in this miniature Story Hour I wrote about it:
http://enworld.cyberstreet.com/showthread.php?s=&threadid=29529
Against the Stone Lord - I ran this for a group of friends on a beach trip and it turned into one of the funnest sessions ever - The party is a group of relatively low level adventurers who have returned victorious to town after an adventure. They have collected their reward from the mayor and go to the inn to celebrate with a drink. As they are hoisting their toast, the door burst open and they turn to see who it is...
They feel water dripping down their heads and the room has suddenly become dark and cold. What happened was this: The person coming in the door of the inn was a Medusa and they were all turned to stone. Twelve years have passed since then. In the mean time, the Medusa, now known as the Stone Lord has taken over the surrounding area with her minions (I used bugbears).
The mayor they had previously aided tried to raise enough money to have the party returned to flesh, but failed. The best he could do was a temporary elixer which he applied to them just now (that's why their hair was wet - it was also nightime and winter now, which is why the room got dark and cold). They will retain their flesh forms for one week after which they will become stone again. The only way to permanently cure themselves is to drink a potion made from the blood of a Medusa. The Stone Lord's keep is three days away...
Highlights: The party was immediately faced with a dilemma - they had become "fixtures" at the inn, a visible symbol of what happens if you run afoul of the Stone Lord. Their absence would be noted immediately. So they attacked the garrison of Bugbears in town to assure that they wouldn't be pursued and that word wouldn't be sent immediately to the Stone Lord. They stole some horses too and took off at top speed toward the fortress.
There they met some animated statues (used stats for small Earth Elementals) and the Stone Lord's Minotaur bodyguard before the final confrontation between them and the Stone Lord herself and her familiar...a Cockatrice. The final battle was very fun because I had the petrification effects only affect them for a couple rounds (since they were already under the influence of the short term Stone to Flesh elixer). They defeated the Stone Lord and returned themselves to flesh permanently.
I can provide further details on these if you need them. I'd send you the actual write ups but I don't have them with me and you've got to run this thing tonight.
) Jack the Ripper on a '20's cruise liner (the Mauretania, if you remember it). I was actually chilled by the Keeper's description of the chase scene as we ran from deck to deck, descending deeper into the bowels of the boat, trying to stop the madman who "loves his work"...and finding bloody clues as we went along.... 
