Updated July 2013
Yes, you really can make free phone calls using the Internet! Free internet phone programs let you make free phone calls, usually only to the US and Canada, using special software.
The free phone call applications I list below are PC to phone programs, meaning you can make a free call from your computer to a real telephone number so you don't even need a telephone yourself. Google Voice will even connect your real phone to another real phone for free, making a long distance plan unnecessary.
No matter how it works, it's a free call!
To make a free call using an Internet phone, you might have to download free phone call software or use a soft phone from an Internet phone provider website.
Important: A free Internet phone can not be used to make a 911 or similar emergency call. If you need to call 911, use a traditional land-line or mobile phone, or a VoIP telephone service that's approved for 911 use like Vonage.
1. Google Voice
As of July 2013, Google Voice is the very best way to make free Internet phone calls. With Google Voice, you can make free phone calls, free PC to phone calls (via Gmail and a free plugin from Google), and free PC to PC calls (via Google+ Hangouts).
Google Voice is much more than a way to make free phone calls on the Internet. Google Voice is primarily a way to manage the phone numbers in your life and can be used to intelligently route incoming calls to your free phone number provided by Google Voice to any other phone you have. With Google Voice, you can also get free SMS, make a free conference call, and get free voice mail services.
Free calls you make with Google Voice must be to numbers in the United States or Canada.
Google Voice is available as a web app that you can use in any browser, including mobile browsers (as pictured here). Dedicated apps are available for Android, BlackBerry, and iPhone users.
Free phone calls with Google Voice are limited to three hours. However, you can continue to make a free call to the same number over and over again.
iCall is another tool you can use to make free internet phone calls. With iCall, you can make free calls from PC to phone using their free software.
The computer that you make calls from can be anywhere in the world but for the call to be free, the number you call must be in the United States or Canada. You can't make free phone calls using iCall if you're calling outside the US or Canada.
iCall is available as a free desktop application for Windows, Mac OS X, or Linux and as a free iPhone or Android app. Free phone calls will be disconnected after a short five minutes but you can call the number back again as many times as you want.
A flash-based version of iCall is also available on their website but free calls here are limited to three minutes.
VoipBuster is yet another program you can download and make free internet phone calls with. Many popular world destinations are available for free, including the US, Canada, Australia, Hong Kong, and most of Europe.
Free phone calls aren't limited per-call but per week - you're allowed to make 300 minutes worth of calls over the last 7 days for free. That might sound a little complicated, but for an average user that probably means that most calls won't cost anything.
VoipBuster is available via free MobileVOIP apps for iPhone, Android, BlackBerry, Windows Phone, Windows 8, and Nokia devices.
KNCTR is a free Internet phone call service that allows PC to phone calls anywhere in the US or Canada. I found the call quality to be lacking but you may have better luck.
The greatest thing about KNCTR is that there's no limit to your free phone call length, or the number of free calls you make. The program is advertisement supported... heavily.
KNCTR is only available for Windows.
Free PC to PC Phone Calls
I compiled this list of ways to make free Internet phone calls assuming that you're looking for ways to call "real" telephones. However, if you're only interested in free PC to PC calls, any of the many voice and video chat programs will do that.