Field Drainage - Lowers flood damage
Irrigation -- Reduces drought effects
High Yield Grain - 10% more grain
Fertilizer - 10% more grain
Ploughs - 20% more grain
Steam Tractors - 30% more grain
Insecticide - 20% more grain, reduces swarm chances/damages
Cross Pollenation - 30% more grain
Siege Engines - A large, wheeled battering-ram style device which uses a tree trunk as a ram. Very effective for sieged attacks.
Camoflauge - Helps your soldiers to blend in to the environment. Good for Guerrilla warfare.
Catapults - Capable of hurtling boulders at the enemy. Good for sieges, can also be used in frontal attacks/defenses
Burning Pitch Catapults - Similar to the standard Catapults, but hurtles a fiery, tar-like substance to rain fiery death upon your opponents.
Tiger Pits - Hidden pits which the enemy troops will fall into. Good for Guerrilla warfare, can also be used in frontal defense.
Advanced Subterfuge - Your soldiers have learned how to blend in as serfs in foreign Fiefdoms. Helps them to remain undetected during covert operations, and to identify incoming covert troops.
Cannons - Gunpowder projects a large, iron cannonball at the opposition. Good for frontal defense, sieges, and can be used during frontal assaults.
Field Hospitals - Fewer fatalities in battle
Hand Cannons -- A personal size version of the cannon. Fired by lighting a fuse in the back of the device.
Matchlock Muskets -- The first shoulder-fired weapon. Gunpowder is ignited by striking a lit cloth to the chargeplate.
Flintlock Muskets -- An improvement over Matchlocks, the chargeplate is ignited by the striking of a flint.
Rifled Muskets -- Similar to Flintlocks, with a rifled (rotating grooves) in the barrel. Provides a rotation to the shot, increasing long range accuracy.
Iron Armor - Iron body armor provides a small amount of protection from most weapons.
Steel Armor - Much like Iron Armor, but Steel is twice as hard as Iron.
Mithrill Armor - Like Steel armor, but twice as hard as Steel.
Longbows - An improvement on the standard bow, the added length provides more penetration power.
Crossbows - A short, stout bow mounted on the end of a stock. The stoutness provides more penetration than the Longbow.
Steel bolted crossbows - Crossbows using a steel bolt (or arrow) for added penetration ability.
Reinforced Stone Barricade - Large stone walls which are constructed around the perimeter. Great in preventing siege attacks.
Peripheral Moat - A trench of water surrounding the perimeter. Good for discouraging siege attacks.
Burning Pitch Moat - A trench of pitch which may be set ablaze to discourage siege attacks. Also helps to discourage frontal attacks.
Public Sewage -- Lowers health costs around 10%, increases serf's living areas (so more serfs per acre)
Basic Plumbing -- Makes the serfs a little happier (I think around 2.5% on average?)
Border Crossing Guards -- Will let you close your borders off so no immigrants may move in. You'll get a checkbox on the payments & distributions page asking if you want to close the borders for that year.
Blacksmiths -- Increase tax revenues by 20% due to increased productivity of better tools. Also a requirement for most military stuff.
Lumbermills -- Increases tax revenues by 10%. Also a requirement for some other stuff.
Multi-Unit Housing - Requires Public Sewage, Basic Plumbing, and Lumbermills to acquire; will allow you to host 50% more serfs on your land.
Basic Mining - 10% more taxes
Gunpowder - Requirement for others
Steam Engines - Requirement for others
Churches - Higher serf morale
Organized Cities - 50% increase in # of serfs that can be housed
Printing Presses - 10% cheaper education
Libraries - 10% cheaper education
Schools - 20% cheaper education
Hospitals - 10% cheaper health, 20% fewer deaths
Research Centers - Lower research costs, requirement for others
Fire Brigades - Lower chance of wildfires, less damage when they occur