Whereby is a video chat/conferencing tool that does not require any downloads.
VDO.Ninja is a video chat/conferencing tool used in your browser, no downloads needed.
Jitsi is a free, secure, and open source video conferencing with collaboration functionality, video directly in browse. No downloads required.
Skype is a free web-based communication tool which allows people to video conference, make calls, and instant message.
A video chat and video conferencing service hosted by Google.
Slack is a workplace communication tool, where participants can message each other, create channels/discussion groups, and share tools and files. Slack also integrates with other apps/add-ins and other workplace tools.
Discord is a community-building tool where you can connect over voice, video, and text chat. Discord uses text channels to chat over text, and voice channels where you can use voice and video.