ClickMeeting - Collaborate with partners and clients using ClickMeeting professional web conferencing software. Try it now, FREE!
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Zoom - Equip your team with tools designed to collaborate, connect, and engage with teammates and customers, no matter where you’re located, all in one platform.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
BigBlueButton - Engage Your Online Students. BigBlueButton is a web conferencing system designed for online learning.
Sipdroid - Free and open-source SIP/VoIP Client