Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Org mode - Org: an Emacs Mode for Notes, Planning, and Authoring
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
QOwnNotes - QOwnNotes is a free plain-text notepad with markdown support for Linux, Mac OS X and Windows, that works together with ownCloud Notes.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.