WinTAP

WinTap is a packet sniffer and soft-tap developed to mirror 
packets flowing through an Ethernet interface. It is purely 
based on the NDIS which allows us to sniff packets and rewrite
them to a second interface acting as a soft-tap. WinTap consists
of two components,

    1. NDIS 6.0 based protocol driver.
    2. User mode soft-tap.

These two act in tandem to create a soft-tap, where the 
protocol driver sniffs the traffic and delivers to the user
mode application. User mode application does the redirection
logic and returns the packets to be rewritten to second interface.

Please read the description.html for more details.

WinTAP is licensed under GPL 2; source can be found here: 

https://github.com/newtribesmission/wintap