ISSN

The ISSN (International Standard Serial Number) is an eight-digit number which identifies all periodical publications as such, including electronic serials.

RFC 3044 "Using The ISSN (International Serial Standard Number) as URN (Uniform Resource Names) within an ISSN-URN Namespace" specifies how to use ISSNs as URNs, i.e., use the prefix "urn:ISSN:".

OCLC

OCLC Worldcat Control Numbers

Resolve to HTML by prepending http://www.worldcat.org/oclc/, e.g. http://www.worldcat.org/oclc/32210789

OCLC are used by BHL, for example this MARC record for Steppan 1995 http://ia360641.us.archive.org/0/items/revisionoftribep80step/revisionoftribep80step_marc.xml

SICI

SICIs are used by JSTOR.

For more background see http://en.wikipedia.org/wiki/SICI.