I use
Discord for my VoIP. It's free, simple, lightweight, all that good stuff, and I'd highly recommend it to anyone needing a simple program that does the job well, and a little extra.
But if you're looking for something with more features,
this list popped up on Google and they look like they've compiled a half-decent list, although it's just as likely that the companies paid them to put them on the list.
I've been thinking about VoIPs recently, so I'll follow this thread as well.