Good Packet Sniffer – Troubleshooter

I have a network that all of a sudden is extremely slow. I am having difficulty determining what the source of the slowdown actually is. I have looked over all the event logs and such on all the workstations, and servers and I dont find anything worth noting. I think I might need some sort of packet sniffer to see what traffic is actually flowing. i am wondering if anyone can suggest some tools that might help diagnose my problem. Thanks a lot.