GeoNetwork Opensource and the Esri Geoportal Server are two open source and free technologies that can be used to build Catalog Services for spatial information, and are both a great start.
However, I think both could do with more work on usability.
For example, Catalog Services from the Esri Geoportal Server can be accessed from ArcGIS for Desktop to discover OGC WMS services. There is also an open source CSW Client to have ArcGIS for Desktop bind to and display those WMS services that has been shown to work at Is there public OGC Catalog Service (CSW) available which lets ArcGIS for Desktop with CSW Client Add (WMS) To Map?.
On top of technological issues there are of course administrative issues around who can afford to create and maintain a comprehensive catalog of OGC and other GIS web services. I suspect such a Catalog will need to be a federation of catalogs rather than a centalised monolith.
As an aside, I posted an ArcGIS Idea recently to encourage Esri to Publish public test URLs for each OGC web service supported by ArcGIS but it is yet to take off.