public tools:  passive 

snoop 
ships with solaris
supports filter patterns

netramet
RTFM flow meter	
Coral interface
lots of supporting utilities
used for accounting in NZ, elsewhere

tcpdump
LBL
ships with free Unices
supports filter patterns
understands many network types
well patched, supported, leveraged, evolved
2 tools:  capture  (pcap) and display (text output)