A simple browser based graph paper editor.
To use the tool, visit https://epswartz.github.io/graphpaper/
Hot tip: You can download the page and run offline, it works just fine.
I did not create the majority of this tool, only the download/upload functionalities and the fill tool. The version I started with is by Tom Maitland, and can be found here: http://tommaitland.net/2012/07/online-graph-paper-tool/ . Tom states that he started with a version from someone called Garrett Bartley (Tom changed from imperial to metric units), but the link he provides gives 404.
This version is open source as are Tom and Garrett's versions.
Special thanks to @DanielSokil for the sessionStorage support that allows user's drawings to persist. :)