"... Designed for interoperability, it publishes data from any major spatial data source using open standards." (http://geoserver.org/display/GEOS/Welcome).
GeoServer is the reference implementation of the Open Geospatial Consortium (OGC) Web Feature Service (WFS) and Web Coverage Service (WCS) standards, as well as a high performance certified compliant Web Map Service (WMS). GeoServer forms a core component of the Geospatial Web.
See http://geoserver.org for more details and downloads.