Unfinished Business for Icewind Dale
Like its predecessors for Baldur's Gate and Baldur's Gate II, Unfinished Business for Icewind Dale aims to restore content and quests that were omitted from the game as shipped. The additional content in this mod is based on what I can piece together from various game resources and from talking with former developer JE Sawyer on the Obsidian forums.
There are nine components, all of which can be installed independently of one another. I've tried to be a bit vague with the actual content so as not to spoil the restored content; you can always ask for spoilers or additional information on the mod's discussion forum.
The Voice of Durdel Anatha
Skipped if Auril's Bane Installed
This is probably the best known of the missing content, and a slightly different version is available as part of the core content of Auril's Bane. This component restores the Voice of Durdel Anatha, a cursed paladin of Torm forced to wander around Lower Dorn's Deep. The party can free him of his curse.
There was a book that detailed how Malavon created his twin iron golems. The party can possibly deactivate or even turn them against Malavon.
The trickiest restoration, Marketh originally had a ring that allowed him to teleport around Lower Dorn's Deep, making him impossible to catch. This idea was eventually abandoned for a different path altogether. I've tried to restore this quest while keeping many aspects of the Marketh-Ginafae-Malavon relationship as possible, while still restoring the original quest to deal with Marketh's ring.
Expanded Guello/Beorn Quest
When you speak with the deep gnomes, they actually ask for two things--that you kill Shikata and his salamander kin (thus freeing Guello) and also to stop the umber hulks that are digging and searching for their hidden enclave. Guello will now reward you for both tasks instead of just one of them. Note that this also restores a wave of umber hulks in Tarnelm's oubliette, making the battle here a bit tougher.
The third level of Dragon's Eye features Presio and her legions of undead. As the party travels the area, she speaks to the party via her undead lieutenants. Paladins and mages can call her out to single combat through one of these vessels.
Orrick's Rhino Beetle Shield
The duergar Dirty Llew can fashion a shield out of rhino beetle shells for the party; Nym can further enchant the shield. As an alternative, the party can take Llew's shield to Orrick in Kuldahar instead with a slightly different result.
The High Baptist's Flock
Some leftover dialogues and scripting suggested The High Baptist was supposed to have some followers. When you encounter him on the fifth level of the Dragon's Eye, he'll now have a small but "loyal" congregation.
Minor Item Restorations
A handful of minor items were found in the game files that are not actually available in the game. To minimize influence on game balance these items have primarily been added to stores. New items are the Belt of Stones, Briath's Journal, Gauntlet of Valor, Great Halberd +4, Holy Chaos! Deck, Inferno Darts +4, Potion of Clear Purpose, Potion of Greater Resistance, Potion of Resistance, Silver Bracers of Kedl, and The Dire Old Lute of Pellon Kay. High quality arrows, bolts, short bows, light crossbows, and heavy crossbows have also been restored as random drops.
Restored Random Drops
Requires Heart of Winter
The Heart of Winter expansion alters the random drop table to account for new items scattered throughout the game. In the process several items were removed outright, making them no longer available in the game. This component does not alter items that were simply moved around the table (i.e. Bren Muller's Crossbow) but it does restore several items to the random drop table:
Bastard Sword +3: Incinerator, Bhaal's Fire, Demon's Breath, Foe's Fate, Greater Ring of the Warrior, Large Shield of Strength +1, Long Bow +4: Hammer, Long Sword of Action +4, Morning Star +2: Hammer, Morning Star of Action +4, Potion of Arcane Absorption, Potion of Aura Enhancement, Potion of Constitution, Potion of Dissipation, Reinforced Large Shield +2, Ring of Aura Transfusion, Ring of Greater Resistance, Ring of Reckless Action, Ring of the Warrior Thief, Shocking Flail +4, Short Sword +4: Hammer, Sloth, Spiked Long Sword +1, Static Two Handed Sword +4, Studded Leather +2: Shadowed, Studded Leather +4: Shadowed, Studded Leather of Resistance +3, Two Handed Axe of Resistance +3, Two Handed Sword +1: Hammering, Two Handed Sword +3: Bane, Two Handed Sword +4: Backbiter, Two Handed Sword +4: Life Giver, Two Handed Sword of Resistance +1, War Hammer +4: Defender, War Hammer of Phasing +2 (2H), and the War Hammer of Phasing +3.