Skype

It's free and many people use it. It does alright quality calls & alright quality audio, at the desktop.

You can only talk with other Skype users (without other technology or tools to traverse with). It's not that reliable, and when it doesn't work, there's not really anything you can do. I'd rather rely on something that has been designed for professional use, not consumers. Your broadband needs to be good, and can slow down your computer. The video isn't good enough quality to replace face to face meetings, and it isn't very secure for confidential meetings.

I use it for talking with family and friends, but would definitely recommend a paid for solution for business settings, where you have support, integration, interoperability, services and knowledge from the people who know what they are talking about. Try talking to Microsoft, they will just tell you "they're great" but no one can actually answer any questions, and they encourage you to buy more systems & spend more money in your meeting rooms but their technology is overpriced, and not good quality in comparison to many others. It's a free technology. There's a reason there's an industry with paid for solutions, and a "free/freemium" industry, as with any other industry. There's a difference between value and cheap.