What carriers can utilize my unlocked AT&T iPhone 5?
On the back of your phone you will find a 5 character model number, Specifically the letter A followed by 4 digits.  

You stated you have an iPhone 5, and it was original to the AT&amp;T network you likely have model A1428 which will work in the USA on 

    Aio
    Alaska Communications
    AT&amp;T
    Family Mobile
    GCI
    Net 10
    Straight Talk
    T-Mobile
    Union Wireless


If your model number is different or you will be outside the USA, then use this table on the Apple website to look up your specifics.