Nebula - "Nebula is a scalable overlay networking tool with a focus on performance, simplicity and security. It lets you seamlessly connect computers anywhere in the world. Nebula is portable, and runs on Linux, OSX, and Windows."
OpenVPN - "OpenVPN is an open-source commercial software that implements virtual private network (VPN) techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses a custom security protocol that utilizes SSL/TLS for key exchange. It is capable of traversing network address translators (NATs) and firewalls. It was written by James Yonan and is published under the GNU General Public License (GPL)" - OpenVPN Wikipedia entry
Tinc - "tinc is a Virtual Private Network (VPN) daemon that uses tunnelling and encryption to create a secure private network between hosts on the Internet."
WireGuard - "WireGuard is a free and open-source software application and communication protocol that implements virtual private network (VPN) techniques to create secure point-to-point connections in routed or bridged configurations. It is run as a module inside the Linux kernel and aims for better performance than the IPsec and OpenVPN tunneling protocols. It was written by Jason A. Donenfeld and is published under the second version of the GNU General Public License (GPL)." - WireGuard Wikipedia entry
ZeroTier - "ZeroTier, Inc is a software company with a Freemium business model based in Irvine, California. ZeroTier provides open-source tools and commercial products in the SDWAN sector that enable developers, operators and security professionals to create and manage geographically-agnostic virtual data centers. The company's flagship product ZeroTier One is a Private peer-to-peer client that enables devices such as laptops, desktops, phones, embedded devices, cloud resources, and services to securely connect to virtual networks." - ZeroTier Wikipedia entry