I've tried a few options for this. Corkboard.me/NoteApp used to be a good option, but the free tier now limits the number of collaborators to 2 (as other commenters here have mentioned), which makes it unsuitable for Microscope.
Roll20 is good for a shared workspace, but the drawing tools are honestly fairly clumsy compared to any vector drawing app. The built-in voice chat features could be convenient, but overall there was enough friction in creating and manipulating cards that I went looking for a better option.
In this stream the players use draw.io, which is an app built on Google Draw for building flowcharts. This was the best overall experience I had in building diagrams for Microscope. I'm not really a fan of drawing arrows from card to card - to me this creates some visual and mechanical resistance to inserting new cards in between, which is a key part of Microscope - but you don't have to make use of that feature. The grid is convenient, as are the pre-set shapes and extensible canvas. The pre-set color palette is good for setting card tones easily. Anyone with a google account can be a collaborator and edit simultaneously, and google hangouts for voice is pretty frictionless once you're using everyone's google account already.
Overall, out of the tools I've tried, draw.io provided the clearest and most intuitive interface, with all of the tools necessary for playing Microscope in a single convenient view.