Roy Tennant and Owen Stephens compiled an extensive list of library-related APIs.

Library services

• Amazon
• Book Cover Images (LibraryThing)
• Classify (OCLC) - Find Dewey and Library of Congress classification numbers for library items (e.g., books, DVDs, etc.).
• COPAC Search (SRU) and MODS XML fetch by identifer (major University and National Libraries in the UK and Ireland, including the British Library)
• DBPedia - Structured information from Wikipedia
• DOI Resolution (CrossRef) - Requires an account
• Google Book Search API - link to items in GBS, and find out about their availability
• Guess OPAC System (OCLC) - An experimental service that tries to intelligently detect the OPAC vendor and its ISBN,ISSN, and OCLCNUM linking template.
• Hathi Trust - Library-hosted books digitized by Google
• High-Level Thesaurus (HILT) Project - Cross-searching of various controlled vocabularies and terminologies.
• Library of Congress Subject Headings (Library of Congress) - LCSH as linked data using SKOS
• LibraryThing JSON Books API
• LibraryThing Web Services API (LibraryThing)
• Metadata Crosswalk Service (OCLC, Experimental) - Transforms metadata from one format to another.
• OpenCalais (Thomson Reuters) - Creates rich semantic metadata for the content you submit
• Open Library
• OpenURL Gateway (OCLC) - delivers "Where Are You From?" resolver services via portable and institution-independent OpenURL links
• pewbot (Huddersfield University Library) - "People who borrowed this also borrowed that" service
• Scopus (Elsevier) - Search this massive journal article database
• Talis Platform
• Terminology services (OCLC) - Various controlled vocabularies, including LCSH, MeSH, and TGM
• ThingISBN (LibraryThing) - Takes an ISBN and returns a list of ISBNs from the same "work" (i.e., other editions and translations)
• ThingLang (LibraryThing) - Takes an ISBN and returns the language of the book
• ThingTitle (LibraryThing) - Takes a title and returns a list ISBNs from the most likely LibraryThing "work," the LibraryThing title and a link to the LibraryThing work page
• Virtual International Authority File (Library of Congress (LC), the Deutsche Nationalbibliothek (DNB), the Bibliothèque nationale de France (BnF), and OCLC)
• Worldcat Identities (OCLC) - Get information about people/organizations listed in Worldcat
• WorldCat Registry and detail (OCLC) - Find and retrieve basic information about institutions and consortia profiled in the WorldCat Registry, not limited to OCLC members
• WorldCat Search Service (OCLC) - API to the largest bibliographic database in the world
• xISBN (OCLC) - Find all the ISBNs plus edition and other metadata related to a work
• xISSN (OCLC) - Find all the related ISSNs
• xOCLCNUM (OCLC) - Find all OCLC Numbers plus edition and other data related to a work

Individual Library Catalogs

• Mirlyn (University of Michigan)
• North Carolina State University

Directories

• Information Environment Service Registry (JISC)
• ProgrammableWeb.com - Both library and non-library services.

Tools & Specifications

• Atom Publishing Protocol
• DLF ILS Discovery Task Group (DLF-ILS) Technical Recommendation (Digital Library Federation) - "An API for effective interoperation between integrated library systems and external discovery
  applications."
• IndexData Software Tools
• Jangle - An open specification for exposing content (starting with, although not exclusive to, library services) consistently and simply using the Atom Publishing Protocol.
• OpenSRF (Evergreen)
• PatREST and more (including modules to interface w/III) (John Blyberg)
• SRU: Search and Retrieve via SRU (Library of Congress) - Next generation library search and retrieve protocol
• Z39.50 (NISO) - Legacy search and retrieve protocol