"Students of calculus encounter the function sin x/x early in their study, when it is shown that lim x->0 (sin x)/x = 1; this result is then used to establish the differentiation formulas (sin x)' = cos x and (cos x)' = −sin x. Once this has been done, however, the function is soon forgotten, and the student rarely sees it again. This is unfortunate, for this simple-looking function not only has some remarkable properties, but it also shows up in many applications, sometimes quite unexpectedly."

-- Eli Maor, Trigonometric Delights