Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.
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.
Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.
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.
Yak Tack - remember. words.
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.