I know many people use Google Docs for collaborative writing - I used to use Google Docs all the time as well, but OneNote Online is my new favorite! It has a book-like layout and you can create several tabs for different groups so everyone has their own space (their own page in a tab), which is less confusing than Google Docs.

I use Padlet all the time in my class for a quick sentence or paragraph-level group writing activity - it's easy to share with your students and you can embed it very easily! http://padlet.com/salinmiso/8tihmalkbcyl

I have used Poll Everywhere, Padlet, Google Docs/Forms for checking students' group work in class, but I've always wanted a simple tool that takes a second for students to complete and another second for me to check their completion but a tool where students don't get to "cheat" by peeking at what other groups are writing because they can't see what other students are typing until they post their answers!

I love this blog because they have a pretty quick (but helpful!) introduction to all the trendy tech tools with examples and explanations about how to use them in classrooms and they update their posts fairly open!