Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Scribble Online Whiteboard - Scribble Online Whiteboard is an advanced collaborative whiteboard solution that allows you to draw the sketch using the web-based or mobile platforms such as iPad/iPhone and Android.
Sketch - Professional digital design for Mac.
Ryeboard - A practical and visual workspace for modern teams
Affinity Designer - Professional creative software, exclusively for Mac.
Whiteboard Team - Draw, explain and collaborate visually in real-time.