I had a large number of data points (4 million+) that represented depth sounding data referenced by UTM coordinates. I wanted to get a feel for how this data was distributed spatially in reference to a map. Google maps gives the ability to overlay their maps with custom tiles.
This lead me to representing these points by a color legend representing depth and age of data by transparency here. I hosted the data behind those points by dividing it into a fine grid and aggregating the data points into that grid for quick retrieval.
I used Google's App Engine to host the data and it seems to work fine except for the long startup time if the page is accessed after a long time.
The code if you are interested, is in these two open source projects (dsm2-grid-map & gmap-tile-generator)