There are a couple of Pokemon that only evolve when traded with another trainer. Some trade evolutions also require that the Pokemon holds a certain item when traded. Here's the list:

Kadabra - Alakazam
Graveler - Golem
Haunter - Gengar
Machoke - Machamp
Poliwhirl - Politoed (hold King's Rock)
Onix - Steelix (hold Metal Coat)
Scyther - Scizor (hold Metal Coat)
Porygon - Porygon2 (hold Up-Grade)
Slowpoke - Slowking (hold King's Rock)
Seadra - Kingdra (hold Dragon Scale)