Online Code Review Scripts Like Udacity
My company has a training program for software engineering interns. A computer science problem is given to them that they have to code. Every individual intern is going to create a PR to one of our company's repository. Without merging the PR, an instructor can do a code review and add comments, just like Udacity's. IMO, GitHub is better for code review than Udacity. It has everything that Udacity has to offer and much more.

Also, they are learning the GitHub workflow which may be helpful in companies that use GitHub (like mine).