EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.
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.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.