Review: Top 5 VOIP Apps

"Whether you use them for business communication, talking to family over long distances, or just fragging your friends over the latest FPS, communication has been revolutionized by voice calling apps. Here's a look at the five most popular options for having voice conversations over the internet."

Whether it be on your desktop or your mobile device, VOIP applications are becoming more and more popular.  This is a good review of some of the best.  Which one is your favorite, and why?