Riptide Networking is a lightweight C# networking library primarily designed for use in multiplayer games. It can be used in Unity as well as in other .NET environments such as console applications.
It provides functionality for establishing connections and sending data back and forth, leaving it up to you to decide what data you want to send and when. This is ideal if you like to be in control of your code and know what's going on under the hood.
Riptide is 100% free to use under the MIT license and its development is only funded by donations.