www.mathproblems.info 
Mike Shackleford's math pages. I've done the first 106 problems, so feel free
to send me comments 
http://www.qbyte.org/puzzles/ 
Nick's Puzzle pages. This is another wellconstructed site with a variety of puzzles with solutions. 
physics.nist.gov/cuu/Units/current.html 
Definitions of the SI base units (essentially all units of measurement are
defined relative to these units). 
physics.nist.gov/Pubs/SP811/appenB.html 
This is the only repository of conversion factors for units of measurement that
I have ever found, which is both
comprehensive and accurate. Basically, they know which conversions are exact and
they know which units have inconsistent definitions, and they don't introduce
rounding error without telling you. Do not go to www.nist.gov for this info!
