A wordpress plugin that embeds a qrcode for every post in the post itself. That way you can take a picture of the post and get an automatic citation back to it. Auto cite printed material. Is it needed when there are already extensions that send urls from Chrome or Firefox to your phone?
But wait, there's more
This work by MattK is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International
This isn't a full-fledged plug-in, but you can put it into your single post template wherever you'd like the QR Code.
<img src="http://chart.googleapis.com/chart?chs=150×150&cht=qr&chl=<?php echo get_permalink(); ?>" alt="QR Code for this post" width="150" height="150" />