Google Spreadsheets, Google's online, collaborative spreadsheet system in Google Drive has just added an awesome feature - discussions. This feature is already in Documents and Presentations.
It allows you, and the other users of a spreadsheet, to add comments, which can be replied to, to any cell on a sheet. You can even +1 someone to add them to the discussion and send them a notification via email. Another cool feature is that they can reply to the comment directly from their inbox.
This is another great feature that makes Drive and Spreadsheets even more useful.