Terundcastle
Terundcastle is a relatively large mod for TES III: Morrowind that was initially completed in the end of 2004. In 2007, it was modified to fix a few bugs that I found in the mod after I got back to playing Morrowind again.
The mod contains a castle not far from Balmora, in a picturesque location on the outskirts of Ascadian Isles. The castle has 3 interior cells (main 2-storey interior, towers, and the vault). Inside, there are several NPCs with customized dialog, and some Imperial Guards for security. Owner of the castle offers the place for purchase, and once the player agrees, a system of scripts is used to ensure that player gets necessary permission from the Duke and has enough money to purchase the keep. Once formalities are settled, player gets ownership of the keep, items within it become his property, and he/she gets access to the Vault.
Depending on player’s reputation, NPCs from a number of factions will reveal various pieces of information about Terundcastle’s past and current situation.
More information available on this page of Cyrodiil Chronicles:
http://cyrodiil.net/html/terundcastle.html
Download link:
http://cyrodiil.net/files/mods/Terundcastle.esp
Screenshots from TES Construction Set
External look of the keep, part of Balmora is visible:

Main interior:

The vault:

The Vault contains a secret path to a large underground area that leads outside the keep:

Kiranann
Kiranann is the name for a large island located on the Northeast of Vvardenfell. This mod was an ambitious project that was expected to contain a giant Dunmer fortress, one smaller fortress of the same type, an Imperial-styled town, Imperial Legion Fort, and a fishing village. While exterior of Kiranann was completed along with exterior NPCs and fast travel routes, I never got to completing the interiors. The mod now exists as a playable beta version, released in the end of 2003.
Download link:
http://cyrodiil.net/files/mods/Kiranann.esp
Map:

Screenshots from TES Construction Set
Overview of the Northeastern part of the island (smaller island with fishing village, part of the larger island):

Central part of the main island, almost fully covered with the Fortress:

Town of Tiriath and the Legion Keep, as viewed from the docks:

Fortress of Kiranann:

Minor Mods
DomeFix was a small mod to correct a few bugs related to incorrect object placement in unmodded Morrowind. At first, it only fixed the dome on top of the Dunmer fortress of Hlormaren. Later, other fixes were added, but name remained.
Download links:
DomeFix w/o Bloodmoon support — http://cyrodiil.net/files/mods/DomeFix.esp
DomeFix w/ Bloodmoon support — http://cyrodiil.net/files/mods/DomeFix_b.esp
KillerArgo was a small cheater plugin that made Argonians (great race!) almost impossible to defeat in combat. Also added a barrel in Seyda Neen with some cool equipment to take for free. This absurd idea allowed to test some things that are not normally accessible.
Download links:
KillerArgo 1.4 w/o Bloodmoon support — http://cyrodiil.net/files/mods/KillerArgo_1.4.esp
KillerArgo 1.4 w/ Bloodmoon support — http://cyrodiil.net/files/mods/KillerArgo_1.4b.esp