Wireshark. Technically speaking its called a "network analyzer" but in truth its just a packet sniffer. It's the one that the Cisco Corp. makes all their students use to check network traffic across routers.
There are some stipulations with it though. You need to have a nic card that is able to run the program in promiscuous mode so that it can see all the network traffic throughout either interface you're capturing packets across, elaborating on this just a bit basically all info on the net is just broadcast (tunneling aside) and if it isn't meant for your pc for the most part your pc will ignore it. Also if you're running a Linux box you can run it in monitor mode.
I really do suggest looking up some "How to" videos for wireshark as well. That will help you figure out how to use it to its full extent.