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Pylibpcap - A Python module for the libpcap packet capture library. You can use
it to write a custom packet sniffer. [Open Source, BSD-like]

python network library - Dissects network packets, as returned from pylibpcap.
[Open Source, GPL]

pylibnet - Interface for the libnet packet assembly system. [Open Source, GPL]

At least for libpcap there is a Windows port. I know because the win32 flavour
of Ethereal (a GUI packet sniffer) that I use from time to time depends on it.

HTH,
-- 
Gerhard Häring
OPUS GmbH München
Tel.: +49 89 - 889 49 7 - 32
http://www.opus-gmbh.net/