Network forensics is the process of capturing information that moves over a network and trying to make sense of it in some kind of forensics capacity. A network forensics appliance is a device that automates this process.
There are both open source and proprietary network forensics systems available.
Open Source Network Forensics
Commercial Network Forensics
- Code Green Networks Content Inspection Appliance - Passive monitoring and mandatory proxy mode. Simple to us Web GUI. Linux platform. Uses Stellent Outside In to access document content and metadata.
- ManTech International Corporation NetWitness
- Network Instruments 
- NIKSUN's NetDetector
- PacketMotion 
- Sandstorm's NetIntercept - Passive monitoring appliance. Qt/X11 GUI. FreeBSD platform. Uses forensic parsers written by Sandstorm to access document content and metadata.
- Arbor Networks
- GraniteEdge Networks http://www.graniteedgenetworks.com/
- Lancope http://www.lancope.com/
- Mazu Networks http://www.mazunetworks.com/
These systems combine flow analysis, deep analysis, and security event monitoring and reporting.
- Q1 Labs http://www.q1labs.com/
Tips and Tricks
- The time between two events triggered by an intruder (as seen in logfiles, for example) can be helpful. If it is very short, you can be pretty sure that the actions were performed by an automated script and not by a human user.