I found this too much fun not to pass on. Of course, trying to figure out exactly which forum it belonged in was a wee bit challenging...
FreeRice
For every vocabulary question you get right, 10 grains of rice are donated. Imagine that, simultaneously learning, having fun, and doing good in the world. Huh.
And yes, it is ridiculously easy to use this as a procrastination method, but it's intellectually challenging in a way different from anything I've done since high school. Always nice to find a new way to keep the brain sharp.