2 recommended

2
Quip 2.0

Messaging + Documents. Combined in one place.

Mike Coutermarsh48Code @ Product Hunt · Written
Whenever I need to collab on some text, I've been using a Quip doc. Have found it to be the easiest for me. + Commenting features / history are really well done.
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1
Google Docs

Kesava Mandiga3Curious cat. Writer. Explorer. · Written
I like Quip too but Google Docs offers a fairly comprehensive feature set for free and it automatically gives you access to Sheets, Slides and more.
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