Localhost-sinatra is a project mainly written in Ruby, it's free.
my own local sinatra instance to handle requests that otherwise would go off to some ad site
So I read about how to make the internet not suck (as much). And decided to have a go at it.
Things went very well for a long time, until Google Reader tried to load some junk/spam/ad and gave me a nasty error about being unable to connect to the server. To get around that error, I decided to run a local webserver that listens on port 80, handing back an empty string whenever any request for any resource is made.
This project also has a simple osx launchd plist file to be placed in /Library/LaunchDaemons
and initalized via sudo launchctl load /path/to/localhost-sinatra.plist
You can start/stop sinatra by issuing sudo launchctl stop localhost-sinatra or sudo launchctl start localhost-sinatra respectively.