I've used https://meet.jit.si/ for quite some time now... nothing to install, no flash, only needs a plugin if you want to screen-share, works on all platforms.

They were bought by Atlassian recently and they used it to replace the HipChat video/audio chat function.

I'm not sure yet whether you can also use it with the jitsi desktop clients (https://jitsi.org/) seamlessly, but it'd make sense :)

Just saw some news that they also enabled calling into meet.jit.si conferences by phone (https://jitsi.org/news/telephony-support-on-meet-jit-si/) so that's also a bonus point if you liked Skype.