Library of Congress Authorities is a searchable database of LC's Subject, Name (personal, corporate, and geographic), and Title Authority Records. This is the place to find the authorized versions of names and scope notes for subjects.
OCLC's Dewey Cutter Program, for both Cutter and Cutter-Sanborn four-figure tables. The download is free, but is only available for Windows computers. It can be run on Linux with the Wine Windows Emulator.
OCLC Bibliographic Formats and Standards includes a number of standards, most importantly about the usage of each MARC field. Never heard of a 258 field? Want to know what 1st indicator 3 in a 100 field means? Want to see an example 300? Look no further
Ending Punctuation for Variable Fields You're at the end of a 246, and you're not sure whether to end with a period, a dash, and exclamation point... This handy reference won't necessarily explain the reasons behind a particular punctuation, but will tell you which to use.
AUTOCAT is the ultimate cataloging listserv. It's very active; if you have a question about cataloging, it's almost certainly either answered in the list's archives, or answerable by one of the experienced catalogers on the list.