Packet Peeper is a nice network packet analyzer. Also the fact that is free and open-source is very nice and I kindly thank the developer for this. The functionality put forward by the application is enough to analyze the network packets from a power user/developer/administrator point of view. A nice touch is that packets can be captured and displayed in real-time. There is a Streams windows, which synthesize the network connections and the traffic exchanged through the said connections.
Packet Peeper, at least on my iBook G4 1GHz, consumes around 25% CPU time, but the network traffic analyzed is also light (around 100KB/s on receive).