Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
CryptoTE - CryptoTE is a text editor with integrated strong cryptography.
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.
Standard Notes - A safe place for your notes, thoughts, and life's work
Daybreak - An intelligent to-do app that learns when to remind you
OneNote - Get the OneNote app for free on your tablet, phone, and computer, so you can capture your ideas and to-do lists in one place wherever you are. Or try OneNote with Office for free.