[s]damn i thought this thread was about apps that help dealing with procrastination and the like[/s]

For all sorts of individual writing purposes, I use [url=https://evernote.com/]Evernote[/url]. Totally free, its editor has just enough formatting and the big deal for me is that it has sync capability with every device you have Evernote on. Being able to write on my phone when I'm not home is an amazing feature.

For (my admittedly limited experience with) collabs, so far I've used [url=https://docs.google.com/]Google Docs[/url]. Really simple, everyone has an account for it and it serves the purpose just fine.