I was introduced to it by Simon, co-owner of HWI, and acquired my headset last Monday (it is headphones and microphone combined) for just GBP19.99. There are two levels of software. The one I use is 10Mb and you can only call other users of the same software. But, you can chat for as long as you like, to anywhere in the world. And it's free.
The second one is called SkypeOut, which allows you to call any standard telephone or mobile phone anywhere in the world. These calls have to be paid for, but they tend to be cheaper than your usual phone line provider.
To learn more about it, scroll to the bottom of this page and click on the first banner.