Skype is an absolute God send. Back home in Northern Ireland there is a major health problem for our family to deal with and we feel very far away in Paraguay. We have used Skype to ring people and talk things over. It has been so much better than emails.
I also have Google Talk, but none of my family have it installed so they are not in my contacts. A big limitation of GTalk is not being able to ring a telephone - this is where Skype is a killer app, even if you are charged a few pennies to call the UK.
I realized today though that Google have the potential to knock Skype out of the park. They could easily offer free calls to normal phones based on an advertising based business model. In the google maps database there already is the ability to search for a business in a certain area. So Google could have a little directory search in GTalk, search for a pizza resturant in your area - call them for free using GTalk, the advertising has already pad for the call. If you simply enter a number to dial, GTalk could search their database to find the business you are calling. It would be very simply to then offer a list of similar business in the area.
What about calls to personal numbers? These could be offered as a loss leader, or perhaps they could program a simple version of GTalk for all major cell phone OSs then we could call any cell phone with GTalk installed for free - now wouldn't that be something...