What is a robust open source local client HTTP proxy for monitoring and filtering?
The ideal proxy could be scripted and interfaced with at run time.
[UPDATE] A proxy that runs on the client machine and all browsing runs through that proxy
So ... i guess you could run squid on the local machine and force all traffic through that. BUT there really isn't a whole lot you can do to stop someone from just not using that proxy and still getting to the web since the proxy server NEEDS to be allowed to access the internet.
I ask again - what are you trying to accomplish? We can probably give you better answers without knowing the end goal (like a proxy may not be the best solution).
I'm tempted to vote this to SuperUser as it reads like you are looking for a parental control type solution which you would probably get a better response from the people over there.
It's a product. But Komodo IDE (a development environment for a bunch of dynamic languages) does what you're looking for. There's a component built in called HTTP Inspector, you set your application to use it as a proxy (localhost:8080) all traffic runs through it, and you can set up rules to manipulate traffic on the fly. http://activestate.com
I haven't used it myself, but I've heard good things about fiddler: http://www.fiddler2.com/fiddler2/