There is a product by Eltima called Serial to Ethernet Connector which may do what you like, it's at:

http://www.eltima.com/purchase/serial-over-ethernet/

However, the product is quite expensive ($99 per machine) and would not be useful for an application you expect to deploy beyond your own machine. Also, I haven't read in depth on the product, so you may also need their Virtual Serial Port (http://www.eltima.com/products/vspdxp/ also $99) to make a solution like that work. These products are mostly offered to allow businesses, laboratories, manufacturing shops, etc. to continue to use old serial devices in a networking environment. They're geared for professional and manufacturing scenarios, hence their price point.

There are no free products that I know of that can provide this functionality. There are some other less expensive products with varying levels of functionality, but in the end, this is not something you'll easily find for free. Writing one of your own would be out of the question unless you were willing to write a Windows driver (in C++).

I'd strongly suggest that instead you look around for some good tutorials on using sockets. .NET provides a few classes that help abstract the complexity of the Socket classes (TcpListener, TcpClient to name a few) and they are generally not that hard to use, and like the serial port offer their input/output via streams for easy consumption. If you want something that's most similar to a serial port, use UDP - it's "connectionless" and thus it behaves much like a cable - there may or may not be something listening at the other end to receive your data.

I could probably help you get started, if you give me a bit more information about the project you're working on?

fm