OpenLayers

OpenLayers is a pure JavaScript library for displaying map data in most modern web browsers, with no server-side dependencies. OpenLayers implements a JavaScript API for building rich web-based geographic applications, similar to the Google Maps and MSN Virtual Earth APIs. Highly extensible, it serves as the foundation of web mapping interfaces. OpenLayers accesses data through industry standards, leaving it free of server-side dependencies. It is released under a BSD-style license.

Features

• Overlay multiple standards-compliant map layers into a single application.
• Displays tiles/images from WMS, WMTS, TMS, WMS-C, WMTS, Google Maps, Bing Maps, Yahoo Maps, OpenStreetMap, ArcGIS Server, ArcIMS.
• Vector feature rendering and styling with support for KML, GeoJSON, WKT, GML, WFS, GeoRSS.
• Web-based editing, including feature snapping and splitting, via WFS-Transactional (WFS-T) leveraging SVG or VML.
• Pluggable with any JavaScript toolkit (JQuery, Ext JS, Dojo, MooTools).
• Client side map reprojection.
• Feature clustering and paging.

More about OpenLayers:
http://www.openlayers.org/