In Canada - "Buy one, get one free" This is usually used in advertising, so the fewer words the better!

They're both correct sentences but buy one get one free is the one that is used.

In the U.S.: Buy one, get one free.

In the U.S.: Buy one, get one free.

You may come across BOGO - which means Buy One, Get One. This example personally annoys me greatly because it is not correctly describing the transaction, but it is the way things are done in stores in my area of the southern USA.