The Unified Digital Format Registry (UDFR), created and maintained by the California Digital Library, will shut down on April 15, 2016. I don’t know whether the whole site will go away or just the ability to query the registry.
The predecessor project, GDFR, never got to publicly usable status. The site gdfr.info still responds to pings, but apparently not to HTTP requests.
Quoting its description here, so it’s saved in at least one place if the site completely goes away:
The UDFR is a reliable, publicly accessible, and sustainable knowledge base of file format representation information for use by the digital preservation community.
A format is a set of semantic and syntactic rules governing the mapping between abstract information and its representation in digital form. While many worthwhile and necessary preservation activities can be performed on a digital asset without knowledge of its format, that is, merely as a sequence of bits, any higher-level preservation of the underlying information content must be performed in the context of the asset’s format.
The UDFR seeks to “unify” the function and holdings of two existing registries, PRONOM and GDFR (the Global Digital Format Registry), in an open source, semantically enabled, and community supported platform.
The UDFR was developed by the University of California Curation Center (UC3) at the California Digital Library (CDL), funded by the Library of Congress as part of its National Digital Information Infrastructure Preservation Program (NDIIPP). The service is implemented on top of the OntoWiki semantic wiki and Virtuoso triple store.