Tonight I implemented KML overlay support in aprs.fi, which have been requested by some users. You can now place any KML file on top of the real-time map, which opens up a great number of possible applications and mash-ups. The feature is used by adding a 'kml' URL parameter to any real-time map view. Please note that the URL must be encoded properly (/ becomes %2F, : becomes %3A).
For example, here's a simple polygon example made by Google, the Pentagon with a Pentagon-shaped overlay: http://aprs.fi/?lat=38.8720&lng=-77.0570&z=15&kml=http%3A%2F%2Fhe.fi%2Fmisc%2Fpentagon.kml (see the KML file: pentagon.kml)
Here's a KMZ file generated by the Export tool of aprs.fi, stored on another server and overlaid back to aprs.fi: http://aprs.fi/?kml=http%3A%2F%2Fhe.fi%2Fmisc%2Ftest.kmz&lat=60.2326&lng=24.8364&z=12
An external KML file can be used to highlight areas of interest (such as SAR search areas), projecting high-altitude balloon track forecasts, and displaying live data generated by other web services with KML output features. The opportunities should generate lots of fun.