ZeroTier is a secure virtual network backbone that allows multiple machines to communicate as if they were all connected to the same network. It is a peer-to-peer encrypted technology, which means that unlike traditional VPN solutions, messages are sent directly from host to host rather than via a central server or router.
All of the code is open source, and you can host the controller yourself or use the ZeroTierOne service, which has both free and paid plans. I’m currently on their free plan, which is solid, reliable, and consistent.