Digital Library of Mathematical Functions http://dlmf.nist.gov/
This mockup gives an idea of the current ideas about the design and organization of the new Digital Library; these ideas are, of course, subject to change. Content only is only available in a few places.

Math Assistance http://www.ercangurvit.com/
Lists rules and formulas for a number of mathematical subjects, such as plotting graphics, functions, factoring, derivatives, integrals, matrices, vectors, and numerical analysis. In English, French and Turkish languages.