We've launched a website to accompany our "Radar Now!" Android application that shows the usage over the previous day.
An interesting aspect of this custom overlay based map is how the tiles are generated. Since "Radar Now!" pretty much is used in every part of the US, tiles for the entire country have to be generated for each zoom level.
Each night, the tile generator scans through the database for records that have had activity for the previous 24 hours and generates 5 zoom levels in about two minutes.