Twifi is a Mac app that intercepts network traffic (intended for open wifi networks) and filters out certain items. Currently, it displays images opened by others, youtube videos, google search queries. Optionally, it will display usernames and passwords or session IDs. It can also post the authentication details to a twitter account automatically. You'll need libnids to compile this.
I built this at the Kings of Code hackathon 2012 as a fun gimmick and to help raise awareness for the risks involved in the use of public open wifi networks.
This is rough code. Like an angry hippo went to sit on my keyboard several times. So don't take this as my usual coding style.
Although interception of traffic on open wifi networks is legal in most places, because no security is being broken, your use of some or all of these might not be legal in some places. Use at your own responsibility.