Here's an interesting Google Chrome extension/Drive App that has been all over Twitter this week - Voice Comments. Once installed, the app from Learn.ly lets you leave voice comments on a Google Doc. There are some great screencasts of how to install and use Voice Comments.
The one drawback at this point is that the doc loses all formatting while leaving the comment, but they are working on fixing that bug. The comments can be linked directly to specific points in the doc or just as a general comment. This would certainly be a great way to leave feedback to students on their writing! Try it out!
Voice Comments - Google Chrome App