Cloud File Sharing & Collaboration
Google Drive is probably nothing new to you. The popular file sharing app, together with Google Docs, forms a great, free suite of real-time (and async) collaborations tools.
Google Drive itself is a file manager, which makes shared access to files easy. The app also has some extended versioning features, making it easy to revert to older version.
It's sister product, Google Docs, makes real-time collaboration on presentations, documents and spreadsheets easier than ever, and allows you to either directly edit files, or "suggest" changes.