Hi,
I had great experience with JaJah. The advantage to skype or other voip providers is that the other person does not have to be online and you do not have to use a headset. It is totally free if both parties are registered and use the service at least once in a month. I even initiate the calls using internet on my mobile. Jajah rocks.
Also Rebtel is cool to call cheap from one Mobile to another. It sounds mega complicated but once you did it you save massive. I call from the UK to abroad. First I registered with Rebtel - then I entered some phone numbers from my phone book (Austria, Denmark, Germany, Poland etc) and received local London phone numbers for them. When I call a foreign mobile they see a local number (vienna, Copenhagen, Berlin or Warswa) If I ask them to call back whilst I wait on the line the call is totally free. I pay to London the other person pays to his nearest city (usually included in free airtime) and rebtel does not charge.. Have a look yourself
http://www.rebtel.comAlso if you like calling from a computer
http://messenger.yahoo.com/webmessengerpromo.php is a lot cheaper then skype...
Hope this helps
André