Google Scholar - Provides a search of scholarly literature across many disciplines and sources, including theses, books, abstracts and articles. - http://scholar.google.com/
BASE: Bielefeld Academic Search Engine - Searches scholarly internet resources from all academic disciplines. Provides a list of the sources indexed. - http://www.base-search.net/
All Academic - A searchable database of academic publications: journals, working papers, convention papers and magazine articles. Also has an A-Z list of links to online journals. - http://www.allacademic.com/