Practice estimation skills by determining the number of objects, the length of a line, or the area of a shape. Choose the problem type, or choose 'Random' to see a mixture of all three types of estimation questions.

http://www.shodor.org/interactivate/activities/Estimator/

Type in a length and a unit, and this website will convert your boring old unit to something much more interesting, eg 200 cm converts to 39 AA batteries end to end, or 10 five month old human fetuses end to end or even 1.4 standard railway gauges. Fun and educational. Three thumbs up!

http://www.sensibleunits.com/