The College of American Pathologists has just Transferred SNOWMED CT to International Health Terminology Standards Development Organisation (IHTSDO). It is now available for use inside any of the IHTSDO Member countries if you are in the US, you can still get it from National Library of Medicine via UMLSKS.
However you still have to agree to the License Agreement for Use of the UMLS� Metathesaurus. It provides for some very FOSS unfriendly terms… included after the gap.
From the current license:
Some material in the UMLS Metathesaurus is from copyrighted sources of the respective copyright holders. Users of the UMLS Metathesaurus are solely responsible for compliance with any copyright, patent or trademark restrictions and are referred to the copyright, patent or trademark notices appearing in the original sources, all of which are hereby incorporated by reference.
IANAL but it appears that to use SNOWMED you have to link to this some sort of SNOWMED “copyright” document by “reference”. Further it appears easy to mistakenly agree to a patent agreement of some kind by accidentally including some portion of UMLS that is covered by a software patent. I will see if I can find out more.