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.