Sketchboard.io is described as 'Creative team sketching and diagramming on an endless online whiteboard'. There are five alternatives to Sketchboard.io, not only websites but also apps for a variety of platforms, including Windows, Linux, Mac and iPhone. The best alternative is diagrams.net, which is both free and Open Source. Other great sites and apps similar to Sketchboard.io are XMind (Paid), Heimer (Free, Open Source), ThinkSpace : Mind Map & Note (Freemium) and Brainio (Paid).
diagrams.net (previously draw.io) is a free online diagram drawing application for workflow, BPM, org charts, UML, ER, network diagrams. No login or registration are required and features include the ability to save locally (including SVG), a range of stencils, .
Heimer is a desktop application for creating mind maps and other suitable diagrams. It's written in Qt and targeted for Linux and Windows. Features: Easy-to-use UI Very fast Zoom in/out/fit Zoom with mouse wheel Save/load in XML-based .