beep
beep is a pretty new product from what I can find and does not have a video promotion yet. The best I could find was this GIF on their twitter. However, it looks simple and brilliant. If your students are working in Google Docs you can add comments as part of the google programme. What you can't do is add voice notes, an increasing trend for teachers. This is where the beep add-on comes in, it easily allows you to add audio to comments :-) (I am not 100% if beep stays free, I have asked on Twitter)
Floop
Floop is all about feedback and looks really well designed. I have made myself an account and it beautifully synced up with Google classroom. The video here gives a nice overview of what Floop offers which I'm sure will excite many of you. It looks like a nice way for technology to help teachers and students who prefer (or have no choice) to work on paper with feedback.
Onenote
I would think everyone who has looked at Onenote is aware that it can be used to give students feedback. However, have you found all of the different modes of feedback? Take a look at this nice video by the 'Everything OneNote' Youtube Channel for some possible new ideas.
Google
Classroom
If you looked at Beep (top of page) you will already see that in Google you can 'comment' and 'suggest' on student's work. Both really excellent features. You can also build up a comment bank. Above this is the google classroom feedback and grading system that is summarised nicely in this video from the 'GCFLearnFree.org' YouTube channel.
SeeSaw
As with most learning platforms SeeSaw has in-built feedback options. I have included SeeSaw here though as I think they have put more resources into the feedback than many others (they also have a nice video summarising it).