Even with a mixed free + affiliation (for actual data files) access, the ESS website has free and open access, with only some essential links for further inquiries
Access to actual data files requires a free, institution-based registration, while all other areas and files are available to any user
Access to most content is open and free, including more than 220 pdf files ready to download
A specific page lists about a dozen of links, pointing both to internal and external pages/resources