Forlist - A great collaborative to-do list app, worth checking out.

Amie - GitHub for research and data science

Todo - Todo is a to-do and task list app.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

Thought Train - Stop using sticky notes to remember what you're doing 📒 🚫

TiddlyWiki - a non-linear personal web notebook