While I like heat maps, I realize they are often mis-used.

Typically what I've seen is a process whereby the color of each pixel is based on the result of an inverse distance weighted function applied to a collection of points. Any time a map has a lot of overlapping point markers, I think it is worth considering a heatmap.

Here's a web based api.

GeoChalkboard has a good tutorial for it.

You can use IDW in ArcGIS.