STScI (Space Telescope Science Institute) offers public access to its library catalog. Telnet to stlib.stsci.edu and login as user stlib (no password) and follow the menu. A user guide is available on request from firstname.lastname@example.org.
The contents of the ESO libraries at Garching bei München and La Silla can be browsed remotely. Access is via telnet to libhost.hq.eso.org, login as user library. Two user guides are available: ``The ESO Libraries On-line Catalogue in a Nutshell'' and ``The ESO User Guide to the On-Line Catalogue.'' Both are available from the ESO Library in Garching (email@example.com). [Reference: ESO Messenger 74, 1993, p. 37.]
The card catalogs of hundreds of libraries (including the Library of Congress, which can be accessed via telnet to locis.loc.gov) are available over the net. Look into the directory ftp.unt.edu:pub/library. The UnCover system (§5.1.4.) also provides access to several larger US libraries (not specialized in astronomy).
Updates and comments:
In section 5.4 I imagine that there might have been some debate about this but really, the use of the word "card" throughout this paragraph is anachronistic. Library catalogues are only referred to as card catalogues if that is what they are on.
Marlene Cummins, U of Toronto Astronomy Library, firstname.lastname@example.org 1995-Jan-5.