Free? No, unfortunately, you cant do that anymore.
What you are talking about is VoIP (Voice over Internet Protocol)
There are 4 services I would recommend:
Skype -
http://skype.com/Vonage -
http://www.vonage.com/index.php?ic=1Vonage is typically connected to your home phone, but, they have recuperate software-only versions that just use a mic in a computer
Yahoo -
http://voice.yahoo.com/MSN has a similar service, but, I couldn't find the information about it on the internet.
I would really try out Skype, which does have a trial. Skype tends to have better pricing options, and their software is not likely to crash on you. I have used Skype quite a bit for PC to PC calls, and PC to phone calls. (Skype had a thing where they allowed PC users to call phones for free, mainly to get more users faster. It was quite popular.)
For any of these, you will need an analog or USB microphone and a headset. To avoid getting a very annoying echo, do not use your regular computer speakers, they will pick up over the mic.