For quite some time I've been frustrated with much code reviews interferred with my normal workflow. Whereas I usually do everything from the terminal and Vim, in order to review my peers' code I had to open the URL in a browser, use a web interface and even use the mouse at times.
Once I was looking for a way to improve this, and I found a nice Vim plug-in called patchreview. It was good to review patch files, but that was definitely not convenient for GitHub Pull Requests.
So I went on and spent a few Fridays building a plug-in for this: vim-codereview, and oh what a delight! It uses the patchreview plug-in to do all the diff heavy lifting, and it interfaces nicely with GitHub.
What you can do with it is essentially reviewing the code in a nice vimdiff-like interface, commenting on a whole Pull Request and commenting on specific parts of the diff.
I set up a brief demo screencast for you to see how to use it :)