Knotable - Cut the noise out of busy email threads
Scrapbox - A new style of note-taking that lets you create, discuss, and learn together in one self-organizing space.
Penbook - The nicest freehand writing app ✍️
Google Keep - Capture notes, share them with others, and access them from your computer, phone or tablet. Free with a Google account.
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.
Draw.Chat - Anonymous real-time whiteboard with text and video chat.