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.
p0f - passive OS fingerprinting (http://lcamtuf.coredump.cx/p0f.shtml)