Leaflet & Google heatmap demo
http://jachymkellar.github.io/heatmap/
Test apps for comparison with: http://jezekjan.github.io/webglayer/
Leaflet.heat plugin
https://github.com/Leaflet/Leaflet.heat
Google Maps Javacsript API - Heatmap Layer
https://developers.google.com/maps/documentation/javascript/heatmaplayer