Which phones can get wifi Internet service if you don't purchase Internet service through your provider?

I'm going to buy a phone on ebay and then call my provider and have it connected without changing my plan but I want to make sure of which phones are out there that can recieve wifi if I'm in a hotspot before I buy the phone. In order to pick up wifi does it have to have a built in wifi and does anyone know which phones have this? Thanks for any help you can give.