SerialToIP is a simple Terminal Server software providing transparent pipe either in TCP server or TCP client socket mode from TCP/IP to a pre-configured serial port. Implemented in C# for .NET 2.0 so the command line version could work in Mono-runtime as well.
SerialPortNet is an open source serial port communication library for Microsoft .NET. It's written in c#, implements complete .NET 2.0 specifications, and is designed to allow for multiple OSes and add-on 3rd party Serial to Network adapters.