The MARC Standards Home Page is not going to win any awards for snappy Web design. Bookmark it anyway. It provides numerous links to primary MARC resources, including:

MARC Formats

Bibliographic, Authority, Holdings, Classification, and Community Information (each of these in full and concise versions); Translations and adaptations, plus MARC 21 Translators’ Tools for those doing translations of MARC documentation; and even a MARC LITE Bibliographic Format, “a true subset of the data elements in the complete MARC 21 Format for Bibliographic Data” designed as “a quick reference guide to tagging.”

Even if your daily use of MARC is in the context of OCLC, my opinion is that you really should know your way around MARC 21, or at least be able to find data elements in their MARC 21 form. Otherwise you’ll be floundering when examining a MARC record from a source other than OCLC, or when setting or troubleshooting specifications for import into a local system.

MARC Code Lists

Geographic Areas
Relators, Sources, Description Conventions

(I’ve mentioned these in connection with OCLC, which no longer maintains separate lists but links to these.)

… and much more. I might just make particular mention of two useful introductory documents, Understanding MARC Bibliographic and Understanding MARC Authority Records.