I am in need of a virtual network adapter driver for Windows XP SP2+ that has an API, preferably one that supports C#. I need to be able to access the bytes that have been sent over the interface and send bytes over it myself. The general idea of what I am looking for is similar to virtual serial ports like this one http://www.fabulatech.com/virtual-serial-port-control.html but for Ethernet instead of serial.
I am looking for a commercial grade product and would love to hear of any you have seen or tried.
I realize it's not quite the answer you're looking for, there is winpcap, which allows you to capture and transmit packets over an actual network interface. There are bindings available for C#, which makes it useful for your purposes.
You can make use of the loopback interface device, which will allow you to have a local only interface to make use of.