Open Source Linux network analyzer
Which are there? What features do they offer?
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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.
p0f - passive OS fingerprinting (http://lcamtuf.coredump.cx/p0f.shtml)