ISOcat is a central registry for all concepts relevant in linguistics and the domain of language resources, including metadata categories etc.
The ISOcat web interface allows you to search for data categories you are interested in, select the ones you want to use, and collect them in your own Data Category Selection (DCS). You can export this DCS for your own application using several export formats.
ISOcat is no longer developed by TLA. See ISOcat.org for details.