FaceTime: Connect Mac to iPhone
FaceTime is a US $1 app on the App Store. You will need to enter an AppleID into the app to send or receive calls, but the software holds your hand through this process, making a new AppleID if needed for you.

Once you have that installed and running, make sure your contact's iPhone is in the address book and dial them up.

You can call out to AppleID that are used on the Mac and iPod/iPad and you call to a "phone number" to iPhones that are in WiFi.

The nice easy support page for FaceTime on the mac is http://www.apple.com/support/mac/facetime/

A short list of hardware requirements is here.