Zoom
Free for 100 participants, up to 40 minutes
https://zoom.us

WebEx
Free for up to 100 participants, up to 40 minutes
https://webex.com/

FreeConferenceCall.com
Free for up to 1000 participants
https://www.freeconferencecall.com/

FreeConference.com
Free for up to 5 participants
https://www.freeconference.com/

Jitsi
Free for unlimited participants
https://meet.jit.si/

Skype Meet Now
Free for up to 30 participants
https://www.skype.com/en/free-conference-call/

UberConference
Free for up to 10 participants, 45 minutes. 
https://www.uberconference.com/