Skype – Offers both free and paid service. Works on all computers, cell phones, and tablets. You can put up to 25 people on the same conference call for free and share a desktop. With the video feature you can only video conference with one person before you have to pay.
Google Hangout – Works on all computers through a google account, such as gmail and is also available as an app via apple/android. Can share desktop. Can call and video chat with up to 10 people for free.
Facetime – An apple product only. Allows 1 to 1 video conferencing. Is nice but only works with apple.
There are more but these seem to be the ones most people are using. I personally use Skype for everything. Having said that if a user is on another system I have accounts for most of them.
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