For advanced students and researchers, much interesting scientific information is in areas not covered by the ordinary web search engines.
Scientific search engines can index the full text of articles, reports and other documents, allowing wider retrieval or access to otherwise "hidden" materials (this does not necessarily mean, however, that users can freely view this full text online). Such access is generally not provided by traditional bibliographies and article indexes.
Major examples include:
CiteSeerX - Focuses on computer and information science
Google Scholar - Best known scientific search engine; ability to locate free versions of papers anywhere on the Web; can link to local journal holdings
OJOSE (Online JOurnal Search Engine) - Find, download or buy scientific publications in up to 60 databases
Q-sensei - Can suggest alternative search strategies, somewhat limited at this point regarding open access content
Scirus - Huge, powerful service offering extensive coverage of preferred web resources (repositories and bibliographies) and search refinement options