» A Dictionary of Units
A summary of most of the units of measurement to be found in use around the world today (and a few of historical interest), together with the appropriate conversion factors needed to change them into a standard unit.
» Funbrain.com
Measurement game - Game which has several levels of difficulty for both English and metric rulers.
http://www.funbrain.com/measure/
» GCSE Maths: Measurement
Includes length, area and volume and the use of prefixes in units. With worked examples and animations.
http://www.gcse.com/maths/measurement.htm
» How Many?
A dictionary of units of measurement: explanations of the metric system, System Internationale (SI), English customary system, and other modern measuring systems. Plus extensive links to other measurement-related Internet resources.
http://www.unc.edu/~rowlett/units/index.html
» How To Read A Ruler
Article describes all of those little lines and markings.
http://www.woodzone.com/articles/Read_A_Ruler.htm
» Jack's Math
Provides several methods for determining length, width and height.
» Online Units Conversion
Gives equal values for areas, volumes, circle measurements, computer storage, currencies, distance and length, energy, fuel consumption, power, pressure, speed, temperature, time, torque, and weights.