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! 

Coran for BG2
by Olga Silaeva 

Thief, archer and romantic!
A full friendship path and some innuendo to boot!
Banters with all barmaids in Athkatla!
Stays with you until the end!

Coran for BG2 is a One-Weekend NPC created for those players, who, like the mod's author, couldn't get enough of Coran's company, having played with him in Baldur's Gate I. Now Coran will keep you company in SoA and ToB with some light banter and a flask of Elverquisst! 

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