Snort is a libpcap-based packet sniffer/logger which can be used as a 
lightweight network intrusion detection system. It features rules based logging
and can perform content searching/matching in addition to being used to detect 
a variety of other attacks and probes, such as buffer overflows, stealth port
scans, CGI attacks, SMB probes, and much more. Snort has a real-time alerting 
capability, with alerts being sent to syslog, a separate "alert" file, or even
to a Windows computer via Samba.

Packets are logged in their decoded form to directories which are generated
based upon the IP address of the remote peer.  This allows Snort to be used as
a sort of "poor man's intrusion detection system" if you specify what traffic
you want to record and what to let through.

For instance, I use it to record traffic of interest to the six computers in 
my office at work while I'm away on travel or gone for the weekend.  It's 
also nice for debugging network code since it shows you most of the Important 
Stuff(TM) about your packets (as I see it anyway).  The code is pretty easy
to modify to provide more complete packet decoding, so feel free to make 
suggestions.

WWW: http://www.snort.org/