What cell phone carriers will let me use an unlocked iphone 4?

So my friend is going to unlock his old Sprint iphone and give it to me. I'm looking to change phone plans and I was wondering which no contract plans will let me use an unlocked iphone?

What do you mean by unlocked?

Having an unlocked phone means that you can use it with ANY cell company. Thats the whole entire point of it being unlocked! Choose whichever one has the best prices.

You might want to consider OFFICIALLY unlocking it before trying to figure out contract plans. You could find yourself wondering what officially means. You can tell this to your friend so he doesn't waste his money on temporary unlocking services or some that don't work.

Official unlocking, means adding your IMEI in Apple's database.

I strongly recommend iphoneofficial.com - really trustworthy, and they will get you an official unlock, putting your IMEI in Apple's database. Check them out.