A great source of expository mathematics (look for articles from the
American Mathematical Monthly and Mathematics Magazine), but only from
UI machines at JSTOR.
A Florida high school math
teacher Jeff Miller
has spent a great deal of time investigating the earliest appearances
of mathematical terms and symbols. His page has many interesting links.
A hilarious mathematical site is Acme Klein Bottles. This
is run by Cliff Stoll, who has written some interesting books
about computers. (You can find amazon.com yourself without a link!)
A repository of interesting material from the history of
mathematics can be found at Convergence, which
is on the MAA website. This is particularly aimed at teachers who
want to incorporate historical material into the classroom.
Ed Sandifer wrote a column for the How Euler did it,
with forty essays on his techniques.
Add for Fall 17:
http://www.mathcounterexamples.net/
http://www.cambridgemaths.org/news/view/what-is-the-roman-numeral-for-ten/
https://vimeo.com/120223777