Ziteboard - Ziteboard is an zoomable online whiteboard that lets team to collaborate in real-time.
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.
Voynetch - Share your visual notes and graphic recordings.
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.
Call to Idea - A gallery of inspiration for product designers
Standard Notes - A safe place for your notes, thoughts, and life's work