The following sites have previously been listed in the history links, but they are interesting enough that they are repeated here in the reference section so that you do not miss them. This is one of the best all around sites that I have found. It is titled “The MacTutor History of Mathematics Archive.”
This is an award-winning site that links to all sorts of mathematics information.
The following two sites are dictionaries of units of measurement:
This site has many nice links to other sites.
This site is very complete dictionary.