ArcWeb API

From their site: ArcWeb Services is a no-cost Web services mapping API that is available to Web developers wishing to access ArcWeb Services. Commercial Services functionality for personal, noncommercial, and nongovernmental development projects. Public Services is offered through a one-year subscription that provides almost all of the same API functionality of Commercial Services however, it only makes available a subset of the mapping data. Public Services provides personal interest Web developers with SOAP, REST, OpenLS, and mobile mapping APIs for integrating mapping and GIS functionality into Web-based applications.