Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface
Recurrr - Send recurring emails
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.
FollowUpThen - The simplest way to schedule an email reminder