Our friends at RCUK have made the subject classifications they use available direct from the RCUK website.
See the pdf and excel links at the bottom of the page:
We are using these in our fledgling data registry (ePrints based) at the University of Glasgow and may roll them out more widely in our repository. I am also aware of a number of other organisations who are exploring using these subject classifications in their information systems include bespoke research systems and PURE.
These classifications seem popular as they cover a wide range of topics and the language is user-friendly and therefore more accessible to the general public and stakeholders who are not research organisations or funders.
I’ve suggested to RCUK that we think of a method of alerting those using the codes to any changes and also that it might be useful to have a unique id for each subject (we had to add one in our system).