That's what website www.poketalk.com are offering people, but there are a couple of minor catches. You can only call for up to ten minutes at a time and most of the available countries only offer landline to landline calls. That being said all you have to do is register with the website and then get dialling. The cost of the call is paid for through their sponsors and other companies who advertise with them.
Above is a list of the countries poketalk can connect you with, but as you can see only Canada, China, Guam, Hong Kong, Puerto Rico, Singapore, Thailand and the USA cover mobile networks too.