Updated 4/1/2022)
Encoded Archival Standards (EAS) home page - https://www2.archivists.org/groups/encoded-archival-standards-section
Complete EAD 2.0 tag library - http://www.loc.gov/ead/tglib/element_index.html
EAD 3 Webinar - Implementing EAD3: Search and Exploration
DACS to EAD to MARC crosswalk - http://www.archivists.org/publications/DACS_TableC5_Erratum.pdf
EAD / MARC crosswalk - http://www.loc.gov/ead/tglib/appendix_a.html#a3
RLG Best Practices - https://www.oclc.org/content/dam/research/activities/ead/bpg.pdf
Yale Best Practices - https://guides.library.yale.edu/facc/bpgs
Over, Under, Around and Through (OCLC paper) - https://www.oclc.org/content/dam/research/publications/library/2010/2010-04.pdf
ISO 3166-1 (country codes) - https://en.wikipedia.org/wiki/List_of_ISO_3166_country_codes, click "Country codes" radio button, and search by name
ISO 639.2 (language codes) - http://www.loc.gov/standards/iso639-2/php/code_list.php
ISO 8601 (normalized dates) - Summary of ISO 8601 by Markus Kuhn; you can also read ISO's brief explanation or (if you're a glutton for punishment) the full standard
LC subject headings (persname, corpname, subject, geogname, etc) - http://authorities.loc.gov
Art and Architecture Thesaurus (genreform) http://www.getty.edu/research/tools/vocabularies/aat/index.html
Syracuse University Special Collections - http://findingaids.syr.edu
Online Archive of California - http://www.oac.cdlib.org/
Archives Hub (UK) - http://archiveshub.ac.uk/
Joyner Library, East Carolina State - http://digital.lib.ecu.edu/special/ead/
Indiana University/Bloomington - http://webapp1.dlib.indiana.edu/findingaids/welcome.do (click "Browse" for options)
Irish Virtual Research Library and Archive - https://digital.ucd.ie/view/ivrla:1
Yale - http://drs.library.yale.edu (set second drop-down to "Limit to collections")
W3C validator - http://validator.w3.org/#validate_by_input
Sample finding aids: More than 1500 finding aids are online here, both in published form and "raw" EAD so you can see the encoding:
Published form http://library.syr.edu/digital/guides/ead/alpha_list_from_db.htm
Raw EAD code http://scrc.syr.edu/ead/xml/
Student EAD assignment, with instructions - https://library.syr.edu/digital/guides/lavender/ead_assignment.html