Nighthawk (released under the terms of GPLv3):
http://code.google.com/p/nighthawk/
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SharpPcap and PacketDotNet (released under the terms of LGPLv3):
http://sourceforge.net/projects/sharppcap/, http://sourceforge.net/projects/packetnet/ 
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NBug (released under the terms of LGPLv3):
http://www.nbusy.com/projects/nbug
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Requirements:
- .NET Framework 4 (http://www.microsoft.com/downloads/en/details.aspx?FamilyID=9cfb2d51-5ff4-4491-b0e5-b386f32c0992&displaylang=en)
- WinPcap (http://www.winpcap.org/)