NATAS is a great free sniffer (No GUI, but the logs are thorough) if you want to see the raw packets.
I disagree with the SNORT reference. Sure Snort is an IDS by design, but it's an excellent sniffer. And with the new signatures out right now, they do support HTML traffic. Also, if you want to get crazy you can right your own sigs to sniff whatever you want....................
