Extensible external metadata standards?

Are there any standard or popular methods of storing external metadata for many generic purposes? I've heard of XMP but how popular is it besides embedded in JPEG images? I'm looking for a format I can use for anything including text files and compressed archives, especially something with implementations in many languages.

Or d开发者_C百科oes everybody roll their own?

UPDATE

I'd prefer a solution not tied to any specific language or platform.

Have you ever watched a MKV movie?

Extensible Binary Meta Language

There are other implementations and related projects