Got an article to write with a fellow classmate? Need some feedback on a blog post you're putting together? Entri lets you write with a good editor, grab the link, then share it with friends to chime in.
It's not a real-time collaboration tool, like Etherpad and its clones, but that's probably a good thing if you're looking for a specific kind of back-and-forth. You can save your changes manually or let auto-saving do the job. When you're done with what you're writing, you can offer your text up by sharing the URL to your "Entri" and letting your friends sign in through Twitter or as guests. If you're ready to move ahead, you can lock your text while you work on something else or pull your research.
Entri is a free app to use and promises more APIs, WordPress integration and more to come in the near future.