At the Daycare Center south of Goldenrod City on Route 33, you can give a male Pokemon and a female Pokemon to the keepers to breed new Pokemon.

Breed two Pikachus to get Pichu.
Breed two Electabuzzes and you get Elekid.
Breed two Clefairies and you get Cleffa.
Breed two Jigglypuffs and you get Igglybuff.
Breed two Magmars and you get Magby.
Breed a Venonat with a Pinsir to get Pinsir.
Breed a Hitmonchan or Hitmonlee with a Ditto to get Tyrogue.
Breed a Clefairy with a Snubbull to get another Snubbul.
Breed a Gyarados with the Red Gyarados to get a golden Magikarp.

Replace any of the Pokemon with the genderless Ditto if you've only got one of a kind. Ditto will act as a wildcard Pokemon.