I just installed the latest version of Yahoo Messenger. That and a 20 dollar headset lets me make free phone calls to anyone else on my messenger list (though they have to have the correct version of the software and a headset as well–and Yahoo reserves the right to start charging for the service.)
I may be a bit behind the wagon, since eBay just bought Skype and I have a friend who has been using vonage for over two years, but I was quite impressed witht he ease of install and the sound quality. I’ll be interested to see where Yahoo takes this. They already allow you to make calls to external phone numbers, though you do have to pay 2 cents a minute.
Why is this any different than Skype or Vonage? Because:
1) it leverages the investment most folks have in their IM address books
2) when you’re on IM, you sometimes want to have phone conversations to supplement the quick questions that IM is so good at conveying
3) yahoo is a fairly well known name (as is google, who is also getting into the business of VOIP). I don’t have any numbers on relative bases, but from personal experience, the number of folks who IM is greater than the number of folks who use VOIP.
Remember the days when there was such a business as carrying long distance voice traffic? There are rapidly coming to a close, driven by VOIP and cellular phone usage, I think.