Mapbender is a PHP/Javasript based map viewing tool that has recently been accepted by the Open Source Geospatial Foundation as its first oficially approved project. “It provides a data model and interfaces for displaying, navigating and querying OGC compliant map services. The Mapbender framework furthermore provides authentication and authorization services, OWS proxy functionality, management interfaces for user, group and service administration in WebGIS projects.”
Another similar product I found recently is OpenLayers which appears to be a pure JavaScript API, similar to Google Maps API.