Google+ but I think it's worth sharing here as well. Beginning today Google Sheets (spreadsheets in Google Drive) will have a comment-only sharing option. Just like Google Documents and Google Presentations the comment-only setting will allow people to view and comment on spreadsheet without actually changing anything in the spreadsheet itself. Learn how to use comments in spreadsheets in the new documentation available here from Google.
Applications for Education
The comment-only option in Google Sheets could be a great instructional tool. You could share data sets with students and ask them to interpret the data, answer questions about the data, and ask questions about the data through the commenting tool. You can reply to students comments. And all of this happens without changing the original data. This is similar to the way that I use the comment-only option in Google Documents for group discussions of primary source documents.