The Unity networking framework will be getting an update soon-ish...I know most developers opt for customized networking solutions but UNET will probably still be good to know.
Lidgren.Network is a networking library for .NET framework, which uses a single UDP socket to deliver a simple API for connecting a client to a server, reading and sending messages.
netcode.io is a simple connection-oriented protocol built on top of UDP.
Developed by Glenn Fiedler
Basic networking system to connect to your own C# server but a pretty well-written system nonetheless!