CNET reports that you can buy a KnowRoaming sticker for $35 which attaches to your SIM card and download the iOS or Android app that goes with it and roam abroad for much cheaper than your current carrier, without the hassle of buying a new SIM card. Only works if your phone is unlocked.

They have deals apparently with numerous carriers. Sounds like a great alternative to now, having to hold multiple SIM cards. Also gives the option of buying a local number so people can also call you.