You'll have a lot to choose from.
The best I've found are...
tcpdump - http://www.tcpdump.org/
ethereal - http://www.ethereal.com/
snort - http://www.snort.org/
tcpdump is a console app. This is handy when you need to log in from random computes during the day ( like I do at wort ) to see what's going on. ethereal is a *very* nice gui sniffer in my opinion. You have capture and display filters. You can quickly modify your display filter, say to temporarily focus on one host, without having to stop your capture. It also has protocol dissectors for just about every protocol on the planet. ethereal comes with tethereal which is a textbased version. It supports capture files from commercial tools as well. snort is not just a sniffer, actually it's a Intrusion Detection System. but you use it as a regular sniffer ( the snort manual explains this )