Turtl - The secure, collaborative notebook
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
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.
Invision - Prototyping and collaboration for design teams
CherryTree - A hierarchical note taking application, featuring rich text and syntax highlighting, storing data in a single xml or sqlite file.
Sketch - Professional digital design for Mac.