www.freerice.com

A UN sponsored website where for every question you get right 10 grains of rice are donated.
The subjects covered are being added to all the time, but you can choose times tablesⁱ and basic maths (pre-algebra).

I have students practicing their timestablesⁱ each session and keeping a running total of the amount of rice they have earned.

This can spin off into other activities, charting their score for each day, weighing rice to find out how much their scores actually translate to in real rice etc.