For collaborative, simultaneous writing and peer feedback, Google Drive (26)/Docs is still king
Students should know how to convert, export, import and move data seamlessly between apps and devices of all kinds. They should also know how to "print to epaper" and how to open and annotate the documents in various readers.
You can also link Dropbox (40) with dropitto.me (41) to have students turn work in even if they don't have access to Dropbox.
No matter what platform, I want every student to know how to "grab" a screenshot.
Not only is this a cyber safety protection skill, it's also great for turning in work from a mobile device when you just can't figure out how to export