Citation database of scholarly articles spanning the sciences, social sciences, arts, and humanities. Indexing goes back to 1900. This database can also search for articles that cite a particular work or author. Formerly called Web of Knowledge.
Database of electronic preprints covering physics, astronomy, math, computer science, quantitative biology, and statistics. (Preprints are drafts of articles; many will later go through a formal peer-review process and be published in scholarly journals.) ArXiv is also available in the database ADS (Astrophysical Data System).