The best alternatives to Inkspace are Gravity Sketch, MapBoard, and Sketch. If these 3 options don't work for you, we've listed a few more alternatives below.
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MapBoard is a online map with whiteboard doodling tools and as many users as you want! It updates in real time, and is a great way to share and collaborate on ideas that have a location. Currently in alpha :)
Tribby3d is a simple yet powerful cloud based structural engineering software for computing tributary areas, calculate element forces and running load take down analysis of multi-story buildings in 3D. Results can be exported as Excel files.