Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
doo - A smart, simple app for reminders and to-dos
TaskPaper - Plain text to-do lists for Mac
Simplenote - The simplest way to keep notes. Light, clean, and free. Simplenote is now available for iOS, Android, Mac, and the web.
Microsoft To-Do - Task management tool
Laverna - Laverna is a JavaScript note taking application with Markdown editor and encryption support.