GI-CAT is a server that aggregates metadata from other, distributed data stores. It is used to serve as a one-stop shop for datasets. Usually GI-CAT is accessed using client software, such as GI-Go (made by the same folks who made GI-CAT). GI-CAT has accessors (harvesters) for more than 20 different types of services, including ESRI GeoPortal Server's CSW, GeoNetworks CSW and the THREDDS ncISO service. GI-CAT can be accessed via CSW, OpenSearch and several other services.
GI-CAT has the virtue of being very easy to install and configure. It comes as a WAR file that can simply be deployed with Apache Tomcat and this YouTube instructional video shows how to configure GI-CAT in less than 5 minutes.
The NOAA IOOS Modeling Testbed is using GI-CAT to harvest ncISO records from distributed THREDDS Data Servers, and then GI-CAT is queried using the OpenSearch interface to return OpenDAP URLs to Matlab users.
This list is really endless. GI-CAT is accessed using open standards. The following are just some notable examples.