The popular web browser Firefox (which I also personally use, and you can download by scrolling to the bottom of this page and clicking on the Firefox banner) now has a VOIP (Voice Over IP) extension that you can download and install. It has been released by Abbeynet Labs as an un-Official product, but you can play with it now! You are required to sign up for a free account on the site, but it allows you to make free internet calls. PSTN (public switched telephone network) calls do cost a nomial fee of course. Check it out!
source: lucafiligheddu.blogspot.com