As advertised on their website, Padlet definitely is one of the easiest ways to collaborate. It acts as an online bulletin board that you can use to display information for any topic, and students can add to it by 'pinning' their responses. Students don't need to sign in, and they can post to any wall you or they set up.
Google Docs is a part of the Google Apps for Education (GAFE) suite of tools that allow for student collaboration. Students can compose a text document, edit it, add comments, and perform many other tasks in real-time.