I stumbled upon this several days ago on Guy's blog. Figured it would be good to let my readers know about this.
Jajah announced that they will be bringing to users free calls between either, or within the following categories:

Zone 1 : Landline and Mobile lines
USA, Canada, China, Singapore, Hong Kong, Taiwan

Zone 2 : Landlines
Argentinia, Australia, Austria, Belgium, CzechRepublic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Israel, Italy, Japan, Korea South, Luxembourg, Malaysia, Mexico, Netherlands, New Zealand, Norway, Poland, Portugal, Russia, San Marino, Spain, Sweden, Switzerland, Thailand, UK, Venezuela

 How it works is you key in your number and your friend's number. The system will then call both parties and connect them. I guess this works based on the assumption that incoming calls are free on both ends. Pretty neat i'd say. 

There are some terms of fair use that you should read when you sign up though…I guess its a safety measure so that people don't abuse the free service - or end up using it to have 18hour phone calls to your girlfriend(s) everyday. Otherwise, check it out =)