I'll help out with a summary:
This includes civilization, unique unit name, unit it replaces and benefit over the replaced unit. This kind of list should more accurately reveal where each unit's speciality and often neglected drawbacks lie. Corrections welcome! List is based on Civ 4: Beyond The Sword 3.0.0.1 (build 81539) data from Civilopedia.
America
Navy Seal - Marine
+1-2 first strikes, starts with March
Arabia
Camel Archer - Knight
+15% chance to withdraw, doesn't need horse and iron
- needs Archery
Aztecs
Jaguar - Swordsman
+Starts with Woodsman, 5 reduced cost, doesn't need iron
-1 Strength
Babylon
Bowman - Archer
+50% vs. melee units
Byzantium
Cataphract - Knight
+2 Strength
- No immunity to first strikes
Carthage
Numidian Cavalry - Horse Archer
+50% vs. melee units, starts with Flanking I
-1 Strength
Celts
Gallic Warrior - Swordsman
+Starts with Guerilla I, can be built with copper
China
Cho-Ko-Nu - Crossbowman
+1 first strike, causes collateral damage
Dutch
East Indiaman - Galleon
+2 Strength, +1 cargo space, can explore rival territory
Egypt
War Chariot - Chariot
+1 Strength, immune to first strikes
England
Redcoat - Rifleman
+25% vs. gunpowder units
Ethiopia
Oromo Warrior - Musketman
+1 first strike, immune to first strikes, starts with Drill I and Drill II
France
Musketeer - Musketman
+1 movement
Germany
Panzer - Tank
+50% vs. armored units
Greece
Phalanx - Axeman
+100% defense vs. Chariot
Holy Roman Empire
Landsknecht - Pikeman
+100% vs. melee units
Inca
Quechua - Warrior
+100% vs. Archery Units, starts with Combat I
India
Fast Worker - Worker
+1 movement
Japan
Samurai - Maceman
+2 First Strikes
- Can't be built with copper
Khmer
Ballista Elephant - War Elephant
+Targets Mounted units first in combat outside cities
Korea
Hwacha - Catapult
+50% vs. melee units
Mali
Skirmisher - Archer
+1 Strength, 50% chance for additional first strike
Maya
Holkan - Spearman
+Immune to first strikes, doesn't need copper or iron
Mongolia
Keshik - Horse Archer
+1 First strike, ignores terrain movement costs
-Isn't immune to first strikes
Native Americans
Dog Soldier - Axeman
+50% vs. melee units, doesn't need copper or iron
-1 Strength
Ottomans
Janissary - Musketman
+25% vs. archery, mounted and melee units
Persia
Immortal - Chariot
+50% vs. archers, receives defensive bonuses
Portugal
Carrack - Caravel
+1 cargo space, can transport military units
Rome
Praetorian - Swordsman
+2 Strength
-5 additional cost, no +10% city attack
Russia
Cossack - Cavalry
+50% vs. mounted units
Spain
Conquistador - Cuirassier
+50% vs. melee units, receives defensive bonuses, upgrades to Gunship
-Doesn't count as post-gunpowder unit for ignoring wall bonus
Sumeria
Vulture - Axeman
+1 Strength
-25% less bonus vs. melee units
Vikings
Berserker - Maceman
+10% city attack, starts with Amphibious
Zulu
Impi - Spearman
+1 movement, starts with Mobility