These sources all cover many subject areas. The top ones are for journal articles, then for news, then for books and book-length documents.
Search for journal articles on most topics. This multi-disciplinary database is a good place to start if you don't know where else to begin. (EBSCO)
Search for journal articles on topics in the arts, humanities, social sciences and some math/science. Most journals have volumes/issues dating from the 19th century to within 3-5 years of present day. (ITHAKA)
Search for journal articles on topics related to the arts, humanities, and social sciences. (Project MUSE)
Search a catalog of library holdings from around the world. Find which libraries have copies of a certain book, DVD, or other material. The FirstSearch interface was designed more with librarians in mind. (OCLC)