My OSM node density visualization has just been updated to show the freshest OSM data!
A significant difference from the 2014 edition is that the density is now calculated as OSM-nodes per m² on the ground (as opposed to nodes per projected pixel in the previous version). If you want to learn more about why this change makes a difference, I'd strongly recommend the following article by Christoph Hormann (imagico).
The updated slippy map lets you chose between the layers of different years (always created around end of June of each year) as well as a display of what changed during each 12 month interval. For example you can see that in North America, there were some imports as well as some major import-cleanups going on during the last year:
Last, but not least: you can now find the source code and instructions to generate all of this in the repository on github: https://github.com/tyrasd/osm-node-density/tree/gh-pages/code