General Purpose

• Google.
• Cuil. Cuil’s goal is to solve the two great problems of search: how to index the whole Internet—not just part of it—and how to analyze and sort out its pages so you get relevant results.
• Clusty. Clustering Search Engine.

• Google Scholar, provides a simple way to broadly search for scholarly literature.
• Google Book Serch,  find the perfect book for your purposes and discover new ones that interest you.
• Google Math Directory.
• Google Code Search. Search public source code.
• Google Special Searches, Linux, Microsoft, BSD, Apple Macintosh, etc.
• SCIRUS, for scientific information only.
• Science Citaton Index, Journal Search.
• Science Citation Index Expanded, Journal Seach.

• Zentralblatt Math Database - Provides comprehensive coverage of the international mathematical research published since year1931. It contains articles from about 2,000 journals and serials, book of all publishers, available proceedings of all conferences, and other research papers appearing in any language. 
• MathSciNet, Mathematical Reiview on the Web.
• MacTutor History of Mathematics.
• 2000 Mathematics Subject Classification.
• Math on the Web.