Besides the various trainers, Route 34 houses the Pokémon Day Care Center. In Crystal version only, the man inside will give you an Odd Egg; with enough time, it will hatch into a female "baby" type: a pre-evolved form of one of the Generation 1 Pokémon: Pichu, Smoochum, Cleffa, Tyrogue, Igglybuff or Magby. There is a 12.5% chance that the hatchling will be shiny.
In any of the three versions, the old lady inside will offer to raise one of your Pokémon for you; you can't choose when or how the Pokémon will level up, and you have to pay for each level your Pokémon earns, but it's a good way to rapidly advance a Pokémon like Magikarp, which is pretty useless before it evolves into Gyarados.
Be sure to get the phone number from the last trainer in Route 34, Camper Todd. He'll call you whenever the Goldenrod Department Store is having a sale!
Abra
Rattata
Pidgey
Hoothoot
Snubbull
Drowzee
Jigglypuff
Ditto