I believe every ambitious programmer makes a configuration language at some point, but most either keep it to themselves.

When I was a teen I made something called Nabla:

  * XML-like syntax

  * Schema language
  * Compact binary representation
  * Trivial parser for binary representation 
  * Optionally, simple dynamic programming language on top

Initially made it for my 3d engine scene serialization format, but then used everywhere some non-trivial data format was needed (e.g. anything with nested data structures).