The tah-heatmap I made of the globe is the current featured image on the front page of the wiki:

Maybe I can convince the t@h guys to run a job on their servers to report tile sizes with more granularity than z12. That would make for a nice map at the country-level.

I've also been in contact with stevefaeembra who made some very nice heatmaps using flickr data overlaid on OSM maps. He's going to release the code behind it so others can render their own as well.

Getting access to OSM tileserver request logs and rendering heatmaps for requested areas and comparing them with heatmaps of our data would make for a very interesting project.