The following new API is available for CrossRef members, affiliates, and general public.
From the CrossRef Blog:
CrossRef has released a new Application Programming Interface (API) tool to allow anyone to look up the Registration Agency (RA) for a given DOI or a group of DOIs. CrossRef is not the only organization that assigns DOIs, and each RA provides different services. For example CrossRef’s services include reference linking and discovery services for scholarly documents. Until now it has been difficult to associate a particular DOI with the correct RA.
To find the registration agency, simply add “http://doi.crossref.org/doiRA/”” before any DOI in your browser address bar or program.