Developers can leverage ArcGIS Online maps and tasks along with ArcGIS Web Mapping APIs and ArcGIS Server to build and deploy Internet applications and mashups that include GIS functionality and ArcGIS services. The ArcGIS Web Mapping APIs are a collection of cross-browser and cross-platform client libraries for creating rich Internet and desktop GIS applications.
These APIs are free for non-commercial use.
The Web Mapping APIs include access to ready to use ArcGIS Online services such as street maps, imagery, and GIS services including routing and geocoding. You can also access services from ArcGIS Server and Bing Maps for Enterprise.
You can get started quickly with ArcGIS Web Mapping APIs using online API samples, getting started help, API references, code galleries, and live interactive maps.
See the following resource centers for more information:
ArcGIS API for Microsoft Silverlight