Yahoo Messenger uses net2phone
Deltathree Free calls to the US & Canda and inexpensive international calls
FreeWebCall Free calls to the US, Canada and the UK
HotTelephone Free calls to over 20 countries including the US, Canada and the UK.
MyfreeLD limited to 2 minutes and around the world
MSN Messenger with hotmail or MSN account uses net2phone limited to 5 minutes
Flat Fee 4ecalls Flat Fee, The lowest Priced Calling Plan in the World, For a $10 Monthly Charge you can make an UNLIMITED number of calls to 20 Worldwide destinations!