http://lowestrates.pushline.com
This site allows you to search for the best rates for calling between two locations. You can then buy a phonecard online, get your pin immediately and start making cheap calls from any phone. You can event register your phone number so you don't have to use your pin each time. Having played around with the site, it finds very competitive rates, e.g. USA to UK for 2 cents a minute, USA to Brazil for 5 cents a minue.
What can I say, it impressed me. Let me know what you think.