Virtual serial ports are a convenient way to test software that accesses hardware over serial ports, particularly when that hardware is not available yet. They can also be useful for logging and tracing purposes.
I maintain a simple tool that allows me to open serial ports, and send and receive data. It is developed using C# and works reasonably well on Linux with Mono. That is the tool I used to test the different redirectors described below.
Free Virtual Serial Ports
Free Virtual Serial Ports version 2.11 by HHD Software works on Windows, right up to version 8.1 64-bit. It is easy to create two bridged serial ports, where data sent to one port arrives at the other and vice-versa. I also love Free Hex Editor Neo by HHD Software.
HW VSP3 version 3.1.2 does serial to TCP redirection. It requires a TCP server at the other end. One way to set that up is to use ncat utility that accompanies NMAP. I have had the need to play with TCP so often, I have rolled my own little tool.
I found that HW VSP3 sends additional data when a connection is established. It also tended to corrupt data during large transfers. It works all right for small controlled tests.