UMLS is huge. It is basically a compendium of other ontologies where the metadata had been unified (the ontology's ontology?). It has snomed, mesh, ncithesaurus, and the kitchen sink. This is nice because it let's me deal with one data format, and let's the user subset the ontologies he wants.

In my investigation, it's sometimes is hard to directly go from a concept, say "medicine" to a set of values, say legal medicine codes. However, I think that is something that could perhaps be layered on top without too much pain.

However, this work is not under active development.