The Center for Mathematics and Science Education at the University of North Carolina at Chapel Hill provides an interesting and easy-to-use dictionary of the history and meaning of many measurement terms. Metric, International, and English Customary Systems are included; but there are also explanations of Apgar scoring, hat sizes, radiocarbon year conversion, and many other tables and scales.

A good site explaining surface area. Scroll down the page for quick references to surface area formulas for fundamental geometric objects. The site has links to related topics.