Amy posted about Kaizena in March of 2014. As she said, "Kaizena is a free resource that let's you leave audio comments, text comments or links to resources on Google Docs. What a great way to give feedback to your students! Students can share their work with you through their Google Drive account (all GCISD students have a Google Drive account using their gcisdstudents.net email address and password). Then you open the document in Kaizena (on a computer) and quickly and easily record comments."
Since posting about Kaizena, they have updated a bit so that you can add "groups" or classes based on what you teach.
If you use Google Classroom and Google Docs pretty extensively, this is a really great tool for giving feedback to students or grading. For instructions on how to get started, see Amy's original post!