Every pixel stands for a single zoom-level 13 tile. The color represents the total number of nodes in that tile. Tiles with less than 1000 nodes are not displayed. A "low-res" version of this visualization is also available, which is based on zoom-level 10 tiles: (click to enlarge)
Copying: visualizations © cc-by-sa - source data © OpenStreetMap contributors, ODbL
In case anyone is wondering how these were made: I've used gnuplot to create the maps, after converting the source data with a simple sed-command. Read more about the actual steps and gnuplot-commands.
Slippy Map Version
There is also a slippy map version! Check it out. ;)