ArcGIS Mapping APIs help you to build and embed interactive maps in your website. You can mashup online services, GeoRSS and social media feeds, and tools and widgets to create custom web maps.
The Web Mapping APIs are
In addition, the ArcGIS API for WPF can be used to build desktop applications.
Development with the APIs is free. Deployment is also free under certain conditions. See More Information for further details.