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dhotson.tumblr.com
dhotson.tumblr.com
It's using qgis to aggregate location data into a hex grid and I used MapBox's TileMill to design the maps.
If you're interested in doing this with your own data, MapBox made a tutorial for this kind of technique here: http://www.mapbox.com/blog/binning-alternative-point-maps/
Enjoy! :-)