HeatMapAPI is a Internet based hosted Heat Map rendering service for GIS systems and maps. It takes a list of points (latitude and longitude) with a value, and creates a thematic 2 dimensional heat map. This service is free to use (limited to the number of points), but a license can be purchased for unlimited usage. See HeatMapAPI.com for more details.
There are two supported modes, summation and average. This allows for random point selections as well as more consistent data models where proximity is known and anticipated.
The API renders images on the fly, real-time, and is very fast. It allows for a developer to integrate their data into a solution and change the data on the fly. The API will accept thousands if not hundreds of thousands of points at once. Bandwidth limited to some degree, but its very, very fast.
It works with just about every major GIS system out there too… not just limited to Google Maps.