There is WebODF, which is 100% pure client-side JavaScript.

http://webodf.org/

http://webodf.org/demo/

I never knew about this until now. That looks pretty amazing and I can see all sorts of potential.

There's all sorts of debate above about office 365 (some reads more like slick advertising astro-turf - people really shouldn't bite at that stuff) but this could be great for many web based business automation apps. I can think of a few things I've done in the past this would have been great for. I see it's already being built in to Zafara and Roundcube.

Looks nice, it's a shame it's AGPL though.

Seriously, multiple down votes? I didn't even get to mentioning toe-jam.

Etherpad (http://etherpad.org) is an OSS cloud document editor app. It has a pretty reasonable set of formatting, revision, and import/export options out of the box, and also a large set of plugins (http://static.etherpad.org/plugins.html). Supported import formats are plain, doc, docx, odt, rtf, and html.

There's a demo on the homepage that runs on Sandstorm.io, so you can very easily self-host or use managed hosting via the Sandstorm framework (https://sandstorm.io).

EtherCalc (https://ethercalc.net/, not related to Etherpad) is an OSS cloud spreadsheet app. Demo on homepage, also available on Sandstorm.

Sandstorm has several other office/productivity apps, and in my opinion, is the clearest route to an effective OSS cloud office suite. I am hopeful for LO Online, but tile-based rendering seems like a bad choice. I have no doubt it will be packaged for Sandstorm at some point, but Etherpad is looking much better as of now.

There is also ONLYOFFICE: http://onlyoffice.org