Ghareth
by Jason Compton

The Cowled Wizards will try to squelch the magic of the twinkling of stars, next. -- Ghareth

Version 0.91 now available, fixes the Madeen bug where Ghareth would leave the party and vanish.

A couple hundred new lines, give or take!
A hastily-recorded soundset, plus three other clips!
Almost 20 banters!
A low-level dual-class NPC!
Magic and moralizing!
Almost 16 hours in the making!

Ghareth, a human fighter/mage, is the first One-Day NPC(tm) attempt. Ghareth requires Throne of Bhaal to function.
With the release of V0.91 (bugfix), no One-Day NPC was actually completed in a single day-long period. But we're close.

Hessa
by Bonnie Rutledge

Some people like bunnies and trees. And those people? I like to kill them. -- Hessa

One neutral evil, half-elf thief, level 12, 13 or 15 depending on the party's experience at the first encounter!
One small NPC-related quest!
One small, new quest-related area!
One new soundset of dubious entertainment value!
12 banters and 20-something interjections!

Hessa is a One-Day NPC* for Baldur's Gate 2, an evil half-elf thief, and a preview of things to come in the Surayah mod. Hessa requires Throne of Bhaal to function. Hessa requires starting a new game.

* (Bons cheated and took one week. Consider it Pluto's first One-Day NPC.)

Alassa
by SimDing0

This is the best time. So much can happen during the night! -- Alassa

An evil female thief
A few interjects, epilogue, etc.
...and some banter
A new soundset (actually also done within a day; cheers Sorschana!)
Something that could have been a quest if I wasn't so lazy
A hastily assembled readme

Alassa is a One-Day NPC** for Baldur's Gate 2, an evil thief who knows how to get what she wants. Alassa requires Throne of Bhaal to function.

** (Oh so close, but Sim decided to sleep and finish the following morning.)

Tiax for BG2
by Olga Silaeva 

A comedy-evil gnome!
Banters with all the BG2 NPCs!
Survives Spellhold after all!
The world's first bilingual twenty-four hour NPC!

Tiax for BG2 is Kulyok's one-day interpretation of the dreaded gnomish spellslinger from BG1. No longer must Tiax fall to lesser talents in Spellhold--take him along for the rest of the game! 

Bruce, the Cockney Barfighter was the subject of a One-Day NPC festival. Different authors, armed with nothing but the name of the character, worked to develop the ultimate man's man. To date, three Bruces have been published.

Other One-Day NPCs

Anishai by Barren
Ninafer by Melkor
Cassius by Gabrielle
Allison by Rastor
Mur'Neth by Andyr