I would like to set up a non-transparent proxy server that will selectively modify HTTP requests and responses as it passes them through. The motivation for wanting to modify the data on the fly and not at the original source are not part of the question. I'm hoping there is a solution which allows the modification scripts to be written in a mainstream programming language. I am aware of guides for Squid which allow you to flip images etc on the way through but I was hoping for a slightly more established and less hackish solution. Does anyone know of an established open source project or environment (Any major platform) which would allow me to script in arbitrary modifications to the data being passed through?