If you don’t need cloud-based discovery or mobile clients, or TightVNC are fully free, GPL-licensed servers. They are fast, lightweight, and do not require any license key whatsoever. Many "pro" features of commercial VNC (like clipboard sharing and multiple connections) are available in these open-source forks.
There are several VNC servers available, including: vnc server license key github hot
A cross-platform C library that allows you to easily implement VNC server/client functionality in your own programs. If you don’t need cloud-based discovery or mobile