ESRI have a number of examples of what can be achieved using their new API. Examples and reference for the API can be found here. Using the API you can:
- Display your own maps on top of a Google Maps base map.
- Execute a GIS model and display the results in Google Maps.
- Search for features in your GIS data and display the results on Google Maps.
- Find addresses using your own address locator and display the result on Google Maps.
- Display attributes from your GIS data on the map using the Google Chart API.
- Allows others to add GIS functionality from your server as a Google Mapplet.
The Drive Time Coverage map is an example of a map built using the API. It allows the user to click on the map and visualise areas that are within a 1,2 and 3 minutes drive from that point.
Google Route Profiler
The Google Route Profiler is another example of what can be achieved using the ArcGIS API. This map creates an altitude profile of any route produced by Google Maps' driving instructions.
The Mapperz Blog has a more detailed and knowledgeable write up of the ArcGIS API release.