Live design feedback with RedPen

This whole concept is awesome. I quote:

Red Pen lets you upload your design, share a short URL, and get live, annotated feedback super-fast.

This is how it works: you go to RedPen.io and upload your image. You'll receive a personalized URL, in short format, like http://redpen.io/ng7h1f. You're then invited to fill your name and e-mail address, so the system remembers you:

From second visit onwards the bar looks like this:

You can then start sending it to people you'd like an opinion from. To leave a comment you just tap on the image, preferably on the exact spot/area you want to comment on, because a red dot appears where you click:

Hovering the image reveals all entry points (dots) for existing comments (or threads if you prefer this naming):

If you keep your link open somewhere, you'll even receive live notifications every time a new comment is added:

Hovering a dot opens a popup with all comments in that spot, allowing you to reply to that particular thread:

Go ahead and give it a try, it's pretty awesome, if you ask me.