Web-sniff is a project mainly written in JavaScript, it's free.
View HTTP requests in real-time with node.js
Web-Sniff lets you view HTTP GET requests over wifi in realtime, using node.js and websockets to present the information
Install mxcl's homebrew, then run brew install node
Get npm and the necessary node packages with curl http://npmjs.org/install.sh | sh npm install socket.io jade pcap
To run (sudo is necessary to set adapter to promiscuous mode) sudo -E node server.js
Open your browser to http://localhost:8000 to view the requests