Can anyone tell me a Skype alternative?
I use the website alternativeto.net when searching for alternatives to certain software. It allows you to filter by platform (macOS, Windows, Linux) and also by licence (ie. open-source/commerical-paid/free-as-in-beer). 

Here is their current list of recommendations for alternatives to Skype for macOS and Windows: https://alternativeto.net/software/skype/


Discord
Telegram (no screen sharing currently)
Tox
Slack
Hangouts


There are also web-based alternatives like deadsimplescreensharing.com, which doesn't require any app or plugin to be installed.