Flashphone is a web service which allows you to call from the browser itself. Unlike Skype or Yahoo Messenger you don’t need to install any software, it is browser based.
Flashphone as the name suggest is flash based and it works well. You have to register for the service to use. Limitations of the service are you are allowed to call 3 times a day and for 3 min each call. The service is still in testing, after the beta period you will able to call for free for longer times. Most of the countries are included in the list, check if your country is supported here.