Colasoft Capsa Free is a powerful tool for monitoring and analyzing network traffic (note, the free version can't monitor network wifi devices).
The program is very accessible. Just choosing an adapter and clicking "Start" gets you an attractive dashboard, with graphs showing network utilisation, traffic, and top traffic by protocol and domain.
But that's just the start. Click the Summary tab and you'll see the data behind the charts, the total numbers of IP and MAC addresses used in this session, the various protocols, DNS queries and responses, SMTP/ POP3/ IMAP 4 connections and a whole lot more.
Maybe you want to zoom in? Choosing one of the Conversation tabs - TCP, say - allows you to drill down, see which packets went to/ from which addresses, the packet size, time sent, and more.
Colasoft Capsa 8 Free captures data packets, too, so you're not restricted to summaries. Selecting any of these items displays the individual packets, and you can choose one, view any text it contains (maybe the password in a POP3 exchange, say). There's even a detailed breakdown of the exchange, so for example you might view an IP packet to check its IP flags or TTL value.
There's vastly more here, and real depth to Capsa's functionality. The program doesn't just understand and decode the basic internet protocols, for instance - it supports more than 300 (including VoIP).
Unsurprisingly, considering the full Enterprise version costs $995, the free build has a lot of restrictions. No monitoring of wifi devices, only one network adapter may be monitored, only one capture project can be run at a time, it's limited to 4 hours maximum, only the first 10 private IP addresses will be analyzed, and more.
Capsa Free has more than enough power to be interesting, though. Just note that you must register with Colasoft to get the serial number you'll need to activate the program. The download link will take you to the appropriate page.
Version 9 brings:
Process View and Process Explorer
Optimized bps calculations
Updated versions of Colasoft MAC Scanner, Ping Tool, Packet Player and Packet Builder
A great tool for understanding and diagnosing all kinds of network issues.