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.
This has been updated for use with Unity3D, the normal repository doesn't work with OSX without crashing, so the patches applied here fix that. I'll be working on some performance improvements too, feel free to send PRs for other bugs fixed as the other repo has been abandoned.
To use this in Unity3D just enable the experimental .NET framework.
This makes it work on Windows, OSX and Linux.