Free alternative to Charles Does anybody know a free alternative to Charles (an HTTP debugging proxy), preferably with a GUI and similar features?

Features that I'm interested in:


Inspect HTTP requests/responses (header, body, etc)
JSON syntax highlighting and formatting
Request filter (by host, URL, etc)
Search
Would be awesome if it could render HTML (Charles can't do it)
You can also check out mitmproxy which even has a web interface. You can inspect HTTP request/responses and even filter requests by certain parameters. 

It doesn't render HTML like you asked but it sure gives out the raw response which might be enough. 

Another suggestion I can think of is the developer tools in Firefox which render HTML and also support request filtering. You can obviously inspect HTTP requests in Firefox too.