It is possible to build ArcGIS Server Web applications using pure JavaScript™ APIs powered by backend REST services that are hosted on any ArcGIS Server. No development or deployment license is required on the Web server hosting your application.
APIs are offered
- to embed maps and tasks from any ArcGIS Server
- to combine GIS content with content from other organizations on top of Google Maps base maps.
- to combine your organization’s GIS content hosted in ArcGIS Server with content from other organizations on top of Microsoft Virtual Earth™ base maps.
Further details can be found here.