As A Web Developer, it is frequently helpful to be able to see exactly what's being sent Between your browser and the web server. Having detailed logs/statistics regarding those http conversations can also be useful and being able to easily acquire images, css files, jss files, etc. from a site is also frequently handy.
Full source code is available below along with a pre-built windows binary distribution. Below the zip file links is zproxy's documentation which is included inside 'zproxy.cfg' which comes with both distributions.