Com0Com can bridge two COM ports within a single windows machine.
A project I am working on opens a COM port and waits for data from a micro controller. I am working on windows application first and will build the hardware later. It costs virtually nothing during this stage of design which is a pro, but verifying functionality is tricky without any hardware to talk to. This is where Com0Com comes in.
I use Com0Com to bridge two ports on the windows machine. The application I am working on opens Com2. In the completed design, Com2 would be the link for the application to talk to the hardware micro controller. Since Hardware is not yet available, Com4 is bridged to Com2. With this, I can open up a terminal on Com4 to talk to the application on Com2. In my case, ill use putty to put the same data the micro controller will spitting out in the future. Now the application has something to talk to!