The one I use is called Echolink, at www.k1rfd.com
It allow hams to link repeater, simplex radios and computers. All users are "checked" as to whether they are licensed when the program is downloaded.
Another is Internet Radio Linking Project, at http://www.irlp.net/
. This one only allows linking of repeater to repeater, and the last time I checked was only available for Linux OS.
Another is eQSO, I am not very familiar with tis one other then it allows SWL users some sort of access.
I hope this some help. I imagine there are other programs/systems.
Bob Echolink Node 10297 Reedsburg/Mauston Wi