I think I'll try and contribute for everything I can, starting with the Grass starters.
Bulbasaur:
-Stadium Pokedex: "The bulb-like pouch on its back grows larger as it ages. The pouch is filled with numerous seeds."
All seed-based attacks have twice the PP they normally would.
Ivysaur:
-Gold Pokedex: "Exposure to sunlight adds to its strength. Sunlight also makes the bud on its back grow larger."
Strong sunlight multiplies Ivysaur's Attack and Special Attack by 1.5.
Venusaur:
-Gold Pokedex: "By spreading the broad petals of its flower and catching the sun's rays, it fills its body with power."
Strong sunlight multiplies Venusaur's Attack and Special Attack by 1.5.
-FireRed Pokedex: "A bewitching aroma wafts from its flower. The fragrance becalms those engaged in a battle."
The opponent is unable to use Rage, Outrage, Dragon Rage, Uproar, or Thrash while Venusaur is on the field. Taunt will have no effect. The ability Anger Point will have no effect.
Chikorita:
-Emerald Pokedex: "It waves its leaf around to keep foes at bay. However, a sweet fragrance also wafts from the leaf, creating a friendly atmosphere that becalms the battlers."
Chikorita automatically uses Sweet Scent on entering the field. Contact moves have an additional chance to miss (I'm thinking 30%) after the first accuracy check is made.
Bayleef:
-Gold Pokedex: "The scent of spices comes from around its neck. Somehow, sniffing it makes you want to fight."
Bayleef automatically uses Taunt when it enters the field. Opposing Pokemon cannot switch out.
Meganium:
-Gold Pokedex: "The aroma that rises from its petals contains a substance that calms aggressive feelings."
The opponent is unable to use Rage, Outrage, Dragon Rage, or Thrash while Meganium is on the field. Taunt will have no effect. The ability Anger Point will have no effect.
Silver Pokedex: "Meganium's breath has the power to revive dead grass and plants. It can make them healthy again."
All fainted Grass-types (yours and your opponents') will be revived at 50% of their max HP when Meganium enters the field.
Treecko:
-FRLG Pokedex: "It quickly scales even vertical walls. It senses humidity with its tail to predict the next day's weather."
Treecko uses Rock Climb automatically when it enters the field. You are alerted to the presence of auto-weather inducers and weather-inducing moves on your opponent's team.
Grovyle:
-Sapphire Pokedex: "This Pokémon adeptly flies from branch to branch in trees. In a forest, no Pokémon can ever hope to catch a fleeing Grovyle however fast they may be."
If Exeggutor or Torterra is on your team, Pursuit and trapping moves have no effect on Grovyle.
-Emerald Pokedex: "Leaves grow out of this Pokémon's body. They help obscure a Grovyle from the eyes of its enemies while it is in a thickly overgrown forest."
Grovyle gets +2 evasion if Exeggutor or Torterra is on your team.
Sceptile:
-Ruby Pokedex: "The leaves growing on Sceptile's body are very sharp edged. This Pokémon is very agile - it leaps all over the branches of trees and jumps on its foe from above or behind."
If Exeggutor or Torterra is on your team, opponents get -2 Evasion against all of Sceptile's attacks.
-Sapphire Pokedex: "Sceptile has seeds growing on its back. They are said to be bursting with nutrients that revitalize trees. This Pokémon raises the trees in a forest with loving care."
Sceptile heals any Exeggutor or Torterra in your party by 50% of its max HP when it enters the field.
Grotle:
-Pearl Pokedex: "The shell is hardened soil. Some Pokémon come to peck the berries growing on the trees on its back."
Grotle always holds some sort of Berry in addition to its normal item and gains the ability Harvest in addition to its normal ability.
Snivy:
-BW2 Pokedex: "Being exposed to sunlight makes its movements swifter. It uses vines more adeptly than its hands."
Snivy has Chlorophyll in addition to its normal ability. Vine Whip has an increased critical hit rate (same level as Slash).
Servine:
-Black Pokedex: "It moves along the ground as if sliding. Its swift movements befuddle its foes, and it then attacks with a vine whip."
Servine has permanent +1 Evasion unless the field has been swamped by Surf. It automatically uses Vine Whip when an opponent's attack misses.
Serperior:
-White Pokedex: "They raise their heads to intimidate opponents but only give it their all when fighting a powerful opponent."
Serperior has Intimidate as well as its normal ability. Its attacks have half the normal power against opponents who don't have at least one base stat of 100 or more.
More to come!