Scott Sheppard alerted me to Project//Draw at draw.labs.autodesk.com. I checked it out on FireFox, and although a tad sluggish, worked as a browser-based diagramming program. Think Visio.
Or, think a resurrected Actrix. When I drew a line between two electronic shapes, the connection stayed intact when I moved a shape. Stretching doesn't work well, yet, as the sink hole elongates into an ellipse as the sink shape is stretched.
There are shapes for doing office and floor plans, org charts, and more. Currently just one person can edit a drawing at a time. MMD/FMD is the native format.
Autodesk Does Mac!
Okay, I'm just baiting you. But presumably Project//Draw works on Macs. As well, one of the stencil sets consists of Mac UI shapes.