Tomato firmware - View port 80 traffic
You'll want to upload and install tcpdump for tomato. You'll need to wget the file and host it somehow and then wget it onto your device. Put in /usr/local/bin or some /bin in your $PATH for easy keeping. See link for file: link
Once you get it installed run something like this (modify for proper interface)
tcpdump -i eth0 -n port 80
See tcpdump cheat sheet