http://scholar.google.com/

http://www.scirus.com/srsapp/   
http://www.ojose.com/

http://www.goole.com/   
http://citeseer.ist.psu.e_d_u/
  
http://yagoohoogle.com/
  
http://www.search4science.com/   
http://www.sciseek.com/