XML schema for electric circuits
As of Version 6, the Eagle layout and routing program has been using XML for all of its files, including schematics and board layouts.  You can download a freeware version here, generate a schematic and look at the corresponding .sch file, which will be in XML.

Prior to using XML, their files were all in a proprietary binary format.  Now it is possible for third parties to develop tools &amp; utilities that can read and manipulate Eagles files, providing added value.

Note that even a tiny schematic may generate a ton of XML; I drew a simply RC circuit and the files was 5800 lines long.  The reason for this appears to be that when you specify a part out of a library, Eagle copies the entire library into the schematic file; this has the advantage that you can give the schematic to someone else that may not have the same library, and they can render it properly.

Once the libraries have been accounted for, the description of the circuit itself does not take all that much space; I have a 13-page schematic, several orders of magnitude more complicated than my simple RC test circuit, and the larger file (25K lines) is only about four times as big as the small one, or about 2K lines per page.