How do I bridge two physical serial ports to each other in software (and log the data going across)?

Question

Basically, I want to put my computer in the middle of a serial line and record the conversation going across it. I'm trying to reverse engineer this conversation and eventually emulate one end of the conversation.

Rough Diagram of what I'm trying to do:

Normally, I have this:

__________        __________  
|        |        |        |  
|Device 1|<======>|Device 2|  
|________|        |________|  

I want to do this:

__________     __________     __________  
|        |     |        |     |        |  
|Device 1|<===>|Computer|<===>|Device 2|  
|________|     |________|     |________|  

With the computer in the middle basically bridging the connection between the two devices and logging the data that goes across.

Answers using any programming language are probably useful. Preferably I would be able to do this on either Windows or Linux (or both if someone has a general solution to this problem).