Using the InChI and the new rdf.openmolecules.net website, it is now possible to tag molecules. And if you use Connotea for that, your tags will even show up on the rdf.openmolecules.net website. For example, at the time of writing, methane was tagged with alkanes and gas . The trick I use, is that the rdf.openmolecules.net gives every molecule a unique HTTP URL.