<p>Thanks, Oaklandmom, for the additional link. I've just had the pleasure of making the online acquaintance of the father of the author of that article, in a Mathematical Association of America special interest group on teaching advanced high school students. </p>
<p>To answer the question upthread, to some degree I tried to order the list so that the links for beginning math learners, and learners who might not go on to advanced math study, appear first. Some links for more advanced math learners (that is, learners of more advanced mathematical topics) come later in the list, although the ordering is not perfect and is debatable in any case. Some of the links are very specialized and some very general. </p>
<p>It would be delightful to see more links of this kind. Math courses in college often act as a "filter" in limiting students' choices of further courses in some majors.</p>