I've used an estimation jar in my classroom for several years. My students love it: the competition seems to motivate them and, up until this year, so did the candy treat for the closest estimate! :) This year, I'll try more "inedible" treats like I've seen on TpT and Pinterest (passes to sit with a friend, extra computer time, etc.)
I fill the jar with whatever we're estimating. Sometimes it's something to count, sometimes how many tablespoons of salt or a liquid, sometimes how many feet/yards of yarn I've stuffed into the jar. I often tape a tablespoon to the jar to help students visualize. A pail of blank papers and a pail for each homeroom class completes the arrangement.