Booksellers, libraries and publishers:

A very extensive list of publishers and bookstores with materials in Arabic, materials about the Middle East, and related matters can be found at
http://www.columbia.edu/cu/lweb/indiv/mideast/cuvlm/Pubs.html

The Oman Studies Center maintains a list of "Bookshops recommended for new and second-hand books on Oman, Zanzibar and the Arab World in general."
www.oman.org/bshops.htm

Palgrave has a section for Middle East and Central Asia books.
http://www.palgrave-usa.com/

Cambridge University Library
http://www.lib.cam.ac.uk/

Middle Eastern Studies resources at the Indiana University Bloomington Libraries
http://www.libraries.iub.edu/libmideast/

Middle East & Islamic Studies, Subject Bibliographies & Indexes, at Cornell University.
http://www.library.cornell.edu/colldev/mideast/bibliogr.htm

The Middle East collection of the Hoover Institution Library and Archives, Stanford University.
http://www.hoover.org/hila/middleeast.htm

The Near East Collection at Yale University: Women in the Middle East
http://www.library.yale.edu/neareast/women.html
http://www.library.yale.edu/neareast/index.html

The British Library: Oriental and India Office Collections, "collecting and providing access to material relating to all the cultures of Asia and North Africa and the European interaction with them."
http://www.bl.uk/collections/orientalandindian.html

UCLA Library Collections and Internet Resources in Middle Eastern, Armenian and Central Asian Studies
http://www.library.ucla.edu/libraries/url/colls/mideast/index.html

The Arab Federation for Libraries and Information (requires browser capable of displaying Arabic text)
http://www.afli.org/

The National Library and Archives of Egypt "is a government research and cultural non- profit authority, whose main concern is to render library services throughout Egypt and the Arab The National Library and Archives of Egypt is a government research and cultural non- profit authority, whose main concern is to render library services throughout Egypt and the Arab World." Requires a browser which can properly view Arabic text.
http://www.darelkotob.org/

The Ottoman archives in Istanbul (Basbakanlik Arsivi) catalog of documents "includes fairly substantial summaries of the contents of each document. They are also going to put actual photographs of about 100,000 documents onto the Internet as the first step to putting the entire archive onto the Internet."
http://www.devletarsivleri.gov.tr

Lib-web-cats "is a directory of libraries worldwide. While the majority of the current listings are in North America, the numbers of libraries represented in other parts of the globe is growing. Each listing includes links to the library's website and online catalog."
http://www.librarytechnology.org/libwebcats/

COPAC "provides FREE access to the merged online catalogues of 24 of the largest university research libraries in the UK and Ireland PLUS the British Library."
http://www.copac.ac.uk/copac/