Open Source Linux network analyzer
Which are there? What features do they offer?
What exactly do you need?
Can't believe nobody mentioned tcpdump. Click on the link to see the list of features
ntop is a solution that has been around for a while, can be extended with plugins. Here is a short how-to.
Wireshark (formerly ethereal) is my favorite.
A few more:
A missing tool from the list is
ettercap. A text-based tool to analyse a subnet and perform active or passive scans of the subnet. It can also monitor packets and display streams.
If you need a network traffic analyzer you can use tshark (it's the console version of wireshark).
MRTG for nice pretty pictures and graphs (management p0rn) via SNMP monitoring. does everything i need, but if u dont list your requirements its quite hard to guess what you want.
p0f - passive OS fingerprinting (http://lcamtuf.coredump.cx/p0f.shtml)
If you need a network intrusion IDS/IPS tool, Snort is one such open source tool.