Once you use GeoNetwork to be your catalog service... you can then use Geoserver to serve up KML and link the KML records to Google Earth (OGC compliant)... very neat and cool... You can download Geoserver from
Download GeoServer
GeoServer is an Open Source server that connects your information to the Geospatial Web.
With GeoServer you can publish and edit data using open standards. Your information is made available in a large variety of formats as maps/images or actual geospatial data. GeoServer's transactional capabilities offer robust support for shared editing. GeoServer's focus is ease of use and support for standards, in order to serve as 'glue' for the geospatial web, connecting from legacy databases to many diverse clients.
GeoServer supports WFS-T and WMS open protocols from the OGC to produce JPEG, PNG, SVG, KML/KMZ, GML, PDF, Shapefiles and more. More information on specific features of GeoServer can be found here, and some samples of GeoServer in action are in the gallery.
Geoserver is built on Geotools, the same Java toolkit that udig uses. GeoServer is a truly open community, with a well documented and modular codebase, so don't hesitate to get involved.