I was recently in Australia, at the Sydney Airport. I had no way to call a shuttle that I knew about. I had used Google Voice from the US to call them.
I almost did not think about this, but the airport had free wifi. I used GMail (funded by my Google Voice account) to call them and it went through fine. I took the shuttle to a part of Sydney that most airport shuttles don't service -- the Coogee Beach area.
Anyone can get a free Google Voice account. I added $10 to my account a while back and it's lasted forever -- $.02 per minute to call Australia, for example. Google Voice works by calling you back. When I am at home and need to place a call, I just enter the number in Google Voice, and it calls me back on my landline and places the call for me, for free anywhere in the US and Canada, and $0.02 per minute to places like England and Australia.
Anyone can call free within the US using GMail. If you have a GMail account, just go to www.gmail.com and find the "call" page. It's like using Skype -- you use your laptop to make the call. It's like using Skype, except without the monthly charge that they try to trick you into constantly extending.
I also went to a local shop in Sydney with my cell phone (a quad-band GSM phone). They tell me it is dirt-simple to put in a SIM for a carrier here and start using the phone. I have my choice of several carriers like Virgin Mobile, Optus. The guy gave me some SIM cards at no charge and told me to just sign up on the carrier's website for a prepaid plan.