When you travel abroad, your cellphone rarely works. When it does, you get socked with roaming rates up to several dollars a minute. A simple fix would be to simply switch out your SIM card for a local one. Of course, with US carriers involved, it's rarely that simple. First off, many US carriers, including Sprint and Verizon, use a different standard than the rest of the world, so if you got your phone from them, you're basically screwed. If you own a T-Mobile or Verizon phone, you're in better shape, but you're not out of the woods just yet. You need a phone that can deal with the GSM frequencies that are used in the countries you're traveling to; the frequency used in the US, conveniently, does not correspond to that used in Europe, which in turn does not correspond to that used in much of Asia. Plus, if you bought your phone from a carrier, it's likely to be locked, which means that you can't take out the SIM card anyway.
So you need to either buy an unlocked cellphone that can deal with multiple frequency bands (see Amazon for some reviews) or have your multiple band cellphone unlocked (for Nokias, you can do it remotely; for others, you'll need to send them in). Once you do that, you can swap in a local SIM card at your destination. (You can also find them cheaper pre-trip on eBay.) Owning an unlocked cellphone also means that you can also take it with you if you decide to switch carriers within the US.