I am working on a networking system using .NET sockets for use in the Unity game engine. Everything is made from scratch including the serializer and concurrent classes. In addition to the networked code, I'm also using threading and syncing the networked operations to the main Unity thread.
I also included a CRC32 in the header for an extra data integrity check, along with an XOR encryption in an attempt to waylay tampered packets. Also included are sequence numbers and acknowledge numbers (and a bitmask to check previous acks).
There is still a lot of work and optimizations to do (like implementing RPCs), but for now it can send any type of data (every frame). It can support any number of clients (as long as the server's hardware can handle it).