This link to Google Scholar requires off-campus users to authenticate and yields search results that indicate which articles are available electronically through the Library. To bypass authentication, go to http://scholar.google.com/.
MIT OpenCourseWare (OCW) for mathematics. MIT OCW is a free publication of MIT course materials (i.e., notes, assignments, projects, exams, and online textbooks) that reflects almost all the undergraduate and graduate subjects taught at MIT.
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.