Citebase Search is a semi-autonomous citation index for the free, online research literature. Citebase contains articles from physics, maths, information science, and (published only) biomedical papers. Still in development and therefore both incomplete and noisy, but still potentially useful.
CiteSeerX is a public search engine and digital library and repository for scientific and academic papers with a focus on computer and information science.
- Google Scholar
Vast, searchable database of literature. The data source for Publish or Perish.
- Publish or Perish
Publish or Perish is a software program that retrieves and analyzes academic citations. It uses Google Scholar to obtain the raw citations, then analyzes these and presents statistics.
- SciFinder Scholar
Quality chemistry and scientific information.
- Web of Knowledge
(Thomson Reuters) A research platform to find, analyze, and share information in the sciences, social sciences, arts, and humanities.