ActionTrip.com
[8.5/10] Reviewed by Uros Jojic. "For all intents and purposes, this is a very solid expansion pack." Also includes screenshots and user comments.

The Lordz Site - Mods for the Total War Series
Offers Napoleonic mod featuring more than 75 new units, mobile artillery, warships, techtree, buildings, flags, banners, portrait images, game pieces, battle formations etc.

Hellenic Total War
Hellenic mod offers new campaign and battle maps, techtree, units including: Spartan Hoplites, Macedonian Cavalry, Persian Immortals and 14 new factions of ancient Greece: Athenians, Persians, Spartans...

Wes Whitaker's Total Modification Site
Medmod mod offers new graphics, rebalances units and buildings and introduces new concepts into game like standard and compound bows.

Medieval: Total War Viking Invasion
Official site. Includes forum, information, downloads, and screenshots.

KekvitMod
Futures playable Irish and Scottish factions, new units, province changes and other improvements.