Lightbeam for Firefox - Mozilla is the not-for-profit behind the lightning fast Firefox browser. We put people over profit to give everyone more power online.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
island - a free and opensource alternative to parellel space
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
AllAccounts - AllAccounts is an extension that allows Firefox to connect to websites using different user names.
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface