I've been working on an plug-in to talk over a Com port to an older Vector drive. I wanted to do some testing before actually powering up the drive and went looking for a free virtual com port driver. I found this: http://www.eterlogic.com/Products.VSPE.html, which looks to be as good as the commercial versions (of which I have used trial in the past).

A virtual Com port driver set up pairs of connect Virtual Com ports on your PC so you can interconnect two programs that are meant to talk over a serial connection. The driver linked above also has some capabilities to redirect a virtual com to TCP. I plan on using it to create a virtual device to test my plug-in with.

interesting link Jeff , looks promising and should save a lot of work in the long run

I've found that VPSE program linked above works great. You can even redirect the com port to TCP which is great as it allows me to remote into the controller PC and redirect the serial traffic back to my desktop PC so I can take advantage of my dual monitors, having Mach/VS on the remote display and the serial data in teh other.

I was testing with Hyperterminal before starting on my own serial tester and then found this: http://realterm.sourceforge.net/index.html#downloads_Download, this seems to be super handy for serial debugging and seems to be extensible.