CML
From Just Solve the File Format Problem
Chemical Markup Language (CML) is an XML-based format for encoding detailed information about a wide range of chemical concepts. It has existed in several versions, originally based on a XML DTD and latterly on a XML Schema. Some aspects of v1 are now deprecated. Schemas and other information are now hosted on the CML SourceForge pages.
The Wikipedia page for CML describes several Open Source libraries and tools for creating, reading and processing CML.
The CML file extension is .cml and the MIME-type is chemical/x-cml.