Dana L. Roth has published an overview of the current alternatives. Services currently offering cited reference searching include (and I give the complete list to have a good look at it):

1. Chemical Abstracts/SciFinder/SciFinder Scholar
2. NASA Astrophysics Data System Abstract Service
3. Amazon.com’s ‘Search Inside this Book’ program
4. Scopus
5. Scitation/Spin Web
6. PROLA (Physical Review Online Archive)
7. Citation Bridge (US Patents)
8. US Patent and Trademark Office
9. Google Scholar
10. Optics InfoBase
11. CiteSeer
12. Science Direct
13. PsycINFO
14. IEEE Xplore
15. Spires HEP
16. IOP (Institute of Physics)
17. CrossRef

She concludes her overview:

… Recent developments of ‘competitors’ to the WoS, while interesting and useful for quick links to some citing references, are clearly not a substitute for a comprehensive citation search. WoS currently indexes ~ 8000 journals from the sciences, engineering, social sciences and the humanities, and clearly remains the primary resource for citation searching….

Source: Dana L. Roth,The emergence of competitors to the Science CitationIndex and the Web of Science (pdf 6pp), Current Science Online, Vol. 89,No. 9, 10 November 2005,[http://www.ias.ac.in/currsci/nov102005/1531.pdf]