Understanding MARC Bibliographic
http://www.loc.gov/marc/umb/
When I first started copy-cataloging, the only resources I had were a copy of Understanding MARC Bibliographic and a MARC flip chart from Follett. Understanding MARC Bibliographic is still the best introduction to MARC that I've seen.

MARC21 Tutorial
http://www.lib.usm.edu/~techserv/pdc/marc21_tutorial_ie/
A tutorial to reinforce what you learned in Understanding MARC Bibliographic

ABLE MARC Tutorial
http://www.lili.org/forlibs/ce/able/course8/01index.htm
Reviews the major MARC tags, explains indicators and subfields. There is a brief quiz at the end of each section.

Tag of the Month (Follett)
http://www.fsc.follett.com/clientsupport/community/tagofthemonth/
Features a different MARC tag each month. Gives a brief description of all the major tags, explains first and second indicators for each tag, and gives examples. Also includes sample MARC records for different types of materials. A good way to become familiar with what a good MARC record looks like.

INFOhio Cataloging Resources
http://www.infohio.org/UC/MARCdocsRev.html
Includes 21 Things to Look for in a MARC Record; guidelines for matching, evaluating, and enhancing MARC records; and six INFOhio Tag Tutors -- video lessons on important MARC tags.