Some more options for synchronous collaboration tools to discover and choose from:
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For spreadsheets, LibreOffice Calc has synchronous collaborative editing (see).
Using desktop software for editing complex documents is preferable anyway, as recreating that software inside a web browser is a dirty hack from a software architecture perspective: complex software in interpreted JavaScript, what to expect if not a performance nightmare and a memory hog?
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For documents, GitHub’s programming environment Atom together with Teletype enabled synchronous collaborative editing of plain-text documents. We’d have to write in Markdown and find a way to get a live preview in a second column, like here in Discourse. Markdown is quite ok even for writing long and complex documents (example on this platform) and has the advantage that people cannot create a mess using incoherent formatting. Finalizing the look & feel could be done by one person after import to LibreOffice.
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For databases, Obvibase is pretty interesting. We used it for the Future Makers Nepal project, instead of that ugly hack of using online spreadsheets (like Google Sheets) as an ad-hoc database. It’s not open source software, but run by one friendly programmer guy.
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For raster graphics drawing, DrawPile is an amazing open source cross-platform application that allows synchronous collaboration, including pressure sensitivity that can be used with graphics tablets or certain on-screen styluses (ThinkPad X series Tablets, Apple pen etc.).
For quick collaborative drawing sessions e.g. to illustrate concepts during a conference call, several collaborative sketching tools are available: sketch.io Sketchpad, iscribble.net, twiddla.com and others.
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For vector oriented drawing that allows synchronous collaboration, so far I don’t know of anything that could replace Google Draw.
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For time tracking with multiple users, it does not look bright. The best cross-platform tool that I could find that comes with local data storage and an optional self-hostable server is Watson, with the server-side companion Crick. This is however a command-line tool. The situation is so dire that my brother started an open source project recently called tom, with the user interface in tom-ui. Will be ready to use in 1-2 months.
And for everything else, the Frama services list is quite interesting. These are 33 free software tools that you can use in the version hosted by Frama or download and install yourself. Most are well-known open source applications like Etherpad Lite that were re-branded by Frama for their services. Which is a bit annoying, but anyway, it’s useful.