“Go Metric,” a site by the U.S. Metric Association; there is a lot at this site:
https://usma.org/teach
The following two sites are dictionaries of units of measurement:
This site has many nice links to other sites.
http://www.unc.edu/~rowlett/units/index.html
This site is very complete dictionary.
http://www.ex.ac.uk/cimt/dictunit/dictunit.htm