Wolfram Alpha is a computational knowledge engine. It is not a search engine; rather, it generates output by doing computations from its own internal knowledge base. The best way to get an idea of its scope and capabilities is to view its Mathematics Examples page.
MathWorld™ is the web's most extensive mathematical resource, provided as a free service to the world's mathematics and internet communities as part of a commitment to education and educational outreach by Wolfram Research, makers of Mathematica.
MathSciNet is a comprehensive database covering the world's mathematical literature since 1940. It provides Web access to the bibliographic data and reviews of mathematical research literature contained in the Mathematical Reviews Database. To see the full titles and ISSNs that correspond to abbreviations in Mathematical Reviews, see Abbreviations of Names of Mathematical Serials (PDF).