The fact that many people have a Skype account is the best part :) With less known platforms, sometimes you'd have to create a separate account in another program, but most have Skype. Also, it is available for almost all the platforms - Apple, Windows, Linux operating systems plus the mobile (native iOS, Android, and WindowsPhone).
It also has video conferencing opportunities and the shared screen feature, which is very useful, especially if you need to show your client a presentation.
You can also easily transfer files (presentations or working documents or pictures) via Skype.
There is another feature: getting a direct link to the group chat that you can send to your colleagues, friends, post it on the internet (in case, for example, you have a webinar coming up that you want people to see and not do it via Facebook or other networks.)
Sometimes the quality of the calls are not the best, but that mostly depends on the internet connection strength.
If you are looking for a product that is available for Mac, Windows, Linux, and Web (and all the major mobile devices) - Skype is your solution. Easy to set up, quite easy to use. Sometimes it has trouble with connection, but most of the time, there's no problem.
1. We keep our clients updated regarding their projects via video/voice calls
2. We meet with potential clients via Skype, especially if they are in another town/country
3. We conduct job interviews over Skype with those candidates, who currently live in another location, yet are interested in the job enough to move and we are interested in their abilities.