Mobile-friendly, minimal, private, and open-source. Works great on your phone as an add-to-homescreen web app, and there is no lock-in with pre-paid hosting.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Fiddler - Fiddler is a debugging program for websites.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Google Tasks - to-do list app that integrates with Gmail and Google Calendar
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.