Mapfluence API from Urban Mapping is a very powerful tool to create heat maps of various information as shown on my web site http://www.elixiraddons.com/urban/zip.php.
There are mainly two pieces. 1) Data determines the info you can display 2) Geometry table determines what level you can show the detail (county, state, zip code, neighborhood etc.).
You can also create queries and process the result using Json. The data are returned in the GeoJson format. The query format is documented on their aite at http://developer.urbanmapping.com/docs
The data is not updated real time and that seems to be the only limitation I can think of.