The Abacus Luis Fernandes. Information on the construction, history, and use of the abacus, as well as comparisons of abacuses from different cultures.
Users can play with an interactive abacus.
American Mathematical Society Information on government affairs and education, publications and research tools, employment, authors, and news in mathematics. Basic help on limits, differentiation, integrals, curve length, dot products, etc. Calculus Tutor Explanations and and problems concerning limits, continuity, derivatives, exponentials, etc. Online help available. Links to other relevant pages.