In addition to freely available Google Scholar and SPIRES, and subscription-based [Web of Science][4] and [Scopus][5], there is a free [NASA Astrophysics Data Systems database][1] which, contrary to its title, appears to have broader scope than SPIRES, at least as far as mathematical physics is concerned; it provides abstracts (but not reviews), citations, and, for many old papers, their full-text scanned versions. Now it is sort of integrated with [arXiv.org][2]: when looking at the abstract of any arXiv preprint, you see the link to its citations and references at NASA ADS under References & Citations. This database has, *inter alia*, a specialized [physics and geophysics search engine][3]. 


  [1]: http://www.adsabs.harvard.edu/
  [2]: http://arxiv.org/
  [3]: http://adsabs.harvard.edu/physics_service.html
  [4]: http://isiknowledge.com/WOS 
  [5]: http://www.scopus.com/home.url