• are comprehensive and thorough in their coverage of the library profession
• are useful to librarians on a daily or nearly daily basis, either for professional development or for performing library work (helping patrons, anwering reference questions, etc.)
• provide news and/or current awareness services for the purpose of "keeping up" with developments in the library field
Now, on to the top ten! (In no particular order . . .)
Internet Library for Librarians
This site has been around since 1994 but is relatively new to me. It's an extremely comprehensive portal site with an incredibly detailed hierarchical index. It covers virtually all aspects of librarianship, includes all the key sites in each category, and each site is annotated. I don't know of another site that more comprehensively covers the field of librarianship, is more detailed, or has a better index.
Digital Librarian: Librariana
By Margaret Vail Anderson, this site provides an alphabetical, annotated listing of the best librariana on the Web. Great for industry news, technology developments, reference work, links to library catalogues, other library portals, subject directories, associations, research papers, opinion pieces, and more. An excellent and eclectic gathering of sites that are great for librarians in many aspects.
The Internet Public Library
The great Teen and Youth divisions, Online Texts Collection in Dewey Decimal classification, excellent Reference Center, subject Pathfinders, Online Literary Criticism Collection, and links to online Newspapers and Magazines/Serials make this an extremely useful site for public librarians and any information professional. The IPL Services for Librarians section covers library employment, advocacy, issues, news, Web organization, and more. A great site.