CiteSeer
Search engine for computer science and related matters. For all results number of citations are given which allows to estimate the importance of a paper in a given context. Most of the papers are cached and available for direct download as postscript or PDF.
http://citeseer.ist.psu.edu/

ISI Highly Cited
A search tool to identify individuals, departments and laboratories that have made fundamental contributions to the advancement of science and technology in recent decades.
http://isihighlycited.com/

iSpecies
A species search engine from Rod Page of Glasgow University. Search on either the common or Latin name of a species. Results are generated by querying other data sources.
http://darwin.zoology.gla.ac.uk/~rpage/ispecies/

Online Journals Search Engine
A multi-search tool that permits queries of a number of scientific journals.
http://www.ojose.com/

Scirus
Search engine that fetches only scientific websites.
http://www.scirus.com/srsapp/

SciSeek
Science search engine and directory. Also provides forums, tools and related news.
http://www.sciseek.com/