Skype for Windows and Mac is well-regarded for offering stable, free VoIP to other Skype users and affordable computer-to-phone calls, to both landlines and cell phones. Call quality still varies depending on your Internet connection and what else you're doing with it while using Skype, although the latest version indicates some progress in ameliorating that through audio engine tweaks.
Since the introduction of free long distance to U.S. cell phones, Skype may have dropped a bit in popularity for domestic calls. For international purposes, though, it's still the standard and extremely popular. Newer features include conference calling with as many as nine people, video calls, the ability to hide incoming call avatars, and a browser plug-in that turns phone numbers into embedded Skype hyperlinks. There are also Skypecasts, throwbacks to the party-line multiperson telephone conversations from the mid-20th century. Whether you're dialing up friends, family, or talking with clients across the globe, it's time to stop calling them and start Skyping them.