Hall.com is a site that I've written about before and it seems like they have been busy in the past couple of weeks. The last time I visited Hall.com, it seemed like a great place to poll your class and provide a chat room type of environment for your students to work. Recently they unveiled their "war room" where users can now chat and collaborate together in real time in a much more powerful way. Some of the new features on Hall.com include group chatting and instant messaging, video chat, file sharing, file collaboration (up to 100 users at once), desktop notifications of @user messages, and more. Check out a screenshot below of the classroom interface:
Hall.com reminds me of a Google Hangout but with a much more "educational" spin. Students can use this space to work together on just about anything and the chat feature enables them to work closely together. This could also be a great way to allow your different classes to work together on one unified project.