DesignExcalidraw
Open-source virtual whiteboard with a hand-drawn feel — 120K GitHub stars
Design
DesignWhimsical
A collaborative visual workspace for flowcharts, mind maps, wireframes, and docs.
Designtldraw
Infinite-canvas whiteboard and SDK — 46,000+ GitHub stars, free web app, $6k/year to ship it in production
ProductivityAFFiNE
Open-source, local-first workspace that merges docs, whiteboard and databases into one hyper-fused canvas
ProductivityHeptabase
Visual, card-based knowledge base for students, researchers, and lifelong learners