Are there less expensive (or free) alternatives to Charles proxy?

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) I’m willing to sacrifice features in exchange for not paying the $50 license fee currently required to license the first seat of Charles proxy suite of tools.