This is the software publisher's description.

The Express Talk softphone works like a telephone to let you make calls through your computer and you can call anyone via the internet who has installed it (or any other SIP softphone). Connect your webcam or other video device and you can see and speak to family, friends and colleagues around the world.

Calls computer to computer are always free. You can also call ordinary 'real' telephone numbers anywhere in the world if you sign up with a VoIP gateway service company. See here for some recommended telephone gateway companies.

Note that this download is for the free consumer version of Express Talk. The business edition will enable you to put people on hold, transfer, record conversations and more. There is also a Mac OS X version for download, although the download here is for the Windows edition.