The Null-modem emulator (com0com) is a kernel-mode virtual serial port driver for Windows. You can create an unlimited number of virtual COM port pairs and use any pair to connect one COM port based application to another. The HUB for communications (hub4com) allows to receive data and signals from one COM or TCP port, modify and send it to a number of other COM or TCP ports and vice versa.
cyclades-serial-client is an RFC 2217 compliant client that works with servers such as Cyclades Terminal Servers and sercd. It creates a virtual serial port which connects to the server over a network.
SWAL is a program that can launch as a service (or standalone) and monitor other services and control other programs. Written in C++ and might have some .NET components (planning on a linux port)