Produced by the Modern Language Association (MLA), the MLA International Bibliography is the definitive index for the study of language, literature, linguistics, rhetoric and composition, folklore, and film, covering scholarly publications from the early 20th century to the present. International in scope, it includes citations to journal articles, books, articles in books, series, translations, scholarly editions, websites, and dissertations.
From the WAC (Writing Across the Curriculum) Clearinghouse, CompPile provides citations to published scholarship in post-secondary composition and rhetoric from 1939 to the present. Its coverage of work from 1939 to 1999 is especially comprehensive.
Google Scholar searches across many disciplines and sources: articles, theses, books, abstracts and court opinions, from academic publishers, professional societies, online repositories, universities and other web sites.