“The DOI (Digital Object Identifier) is an international standard (ISO 26324) managed by the International DOI Foundation, which provides a system to support persistent identification of content objects and related entities in digital networks.”

• https://doi.org
• https://www.crossref.org/services/content-registration/
• https://www.doi.org/the-community/existing-registration-agencies/
• https://www.doi.org/the-identifier/resources/
• doi:10.1000/292
• etc

see crossref

• US based
• pricing https://www.crossref.org/fees/

see datacite

• based in Germany as e.v
• pricing Membership Fee of 2000€ and Service Fee of 500€ per year plus 0,80€ per DOI
• etc+

• mEDRA (multilingual European Registration Agency)
• a brand of ediSer, the service company of the Italian Publishers Association
• pricing (https://www.medra.org/en/terms.htm][details)
  • 150€ startup fee, 180–6000€ per year for 30–8000 DOIs (then 4.50 to 0.45 each)
  • additional 250€ for crossref integration
• guide https://www.medra.org/en/guide.htm

• ONIX for DOI Metadata Schema v2.0

• https://datacite.org/blog/cool-dois/