Pokémon Gold and Silver/Evolution

Evolution
Most Pokémon evolve by leveling up, here are a few that do this:
 * Cyndaquil to Quilava, Level 14
 * Quilava to Typhlosion, Level 36
 * Pidgey to Pidgeotto, Level 18
 * Pidgeotto to Pidgeot, Level 36
 * Spearow to Fearow, Level 20
 * Hoothoot to Noctowl, Level 20
 * Rattata to Raticate, Level 20
 * Sentret to Furret, Level 16
 * Caterpie to Metapod, Level 7
 * Weedle to Kakuna, Level 7
 * Kakuna to Beedrill, Level 10
 * Zubat to Golbat, Level 24
 * Ekans to Arbok, Level 22
 * Poliwag to Poliwhirl, Level 24
 * Meowth to Persian, Level 28
 * Voltorb to Electrode, Level 30
 * Sandshrew to Sandslash, Level 22
 * Tentacool to Tentacruel, Level 30
 * Abra to Kadabra, Level 16
 * Totodile to Crocanaw, Level 18
 * Crocanaw to Feraligatr, Level 30
 * Charmander to Charmeleon, Level 16
 * Charmeleon to Charizard, Level 36
 * Bulbasaur to Ivysaur, Level 16
 * Ivysaur to Venusaur, Level 36
 * Squirtle to Wartotle, Level 16
 * Wartotle to Blastoise, Level 36

Fire Stone

 * Growlithe to Arcanine
 * Vulpix to Ninetales
 * Eevee to Flareon

Water Stone

 * Staryu to Starmie
 * Poliwhirl to Poliwrath
 * Eevee to Vaporeon
 * Shellder to Cloyster

Leaf Stone

 * Gloom to Vileplume
 * Weepinbell to Victreebel
 * Exeggcute to Exeggutor

Thunder Stone

 * Pikachu to Raichu
 * Eevee to Jolteon

Sun Stone

 * Sunkern to Sunflora
 * Gloom to Bellosom

Moon Stone

 * Clefairy to Clefable
 * Jigglypuff to Wigglytuff
 * Nidorina to Nidoqueen
 * Nidorino to Nidoking

Trade

 * Graveler to Golem
 * Kadabra to Alakazam
 * Haunter to Gengar
 * Machoke to Machamp

Trading Holding Items

 * Onix to Steelix holding a Metal Coat
 * Scyther to Scizor holding a Metal Coat
 * Porygon to Porygon2 with an Upgrade
 * Seadra to Kingdra with a Dragon Scale
 * Slowpoke to Slowking with a King's Rock
 * Poliwhirl to Politoed with a King's Rock