Voice Over Internet Protocol - VOIP for Mac - has been around a long time: Even since the days of dial-up modems, since speech doesn't require alot of bandwidth like music or video does. This Mac Gizmo Guy remembers his first 'Free Long Distance' voice phone call 5-some years back on a Slot-Load G3 iMac. I've saved A LOT of money using VOIP on my G5 iMac and MacBook since then.
Skype for Mac
has long been a great money saving option for long distance here in the USA and Canada using low-cost "Skype Out" minutes. The latest Yahoo Messenger v3 for Mac also offers cheap 1 cent/per minute across the nation calls to landlines and cell phones using Yahoo's "Call Out" options. Paired with a dedicated USB or Wireless IP phone - or with a decent headset you can have a great VOIP experience. But in a pinch, even a Mac with a built-in microphone and iSight webcam can get you started.
Other full-fleged OSX VO-IP options are out there with dedicated monthly VO-IP providers like Vonage and others who'll bundle a 'free' Analog Telephone Adapter (ATA) configured for their service as part of your startup package. Just plug in a regular analog phone and you're good to go. Many also offer a SIP - Software Internet Phone application for OSX that lets you make and receive calls directly on your MacBook iMac, Mac Pro or mini.
For those of us who don't want to cough up $100 or more a month for Cable/DSL * 'Unlimited' IP phone bundle packages, or pay the outrageous rates your local phone company may charge - there's a decent range of pay as you go, pay very little by the minute VOIP options for Apple computers that can save you REAL MONEY month after month, call after call.