I discovered FreeRice.com several years ago when I saw my friend post something about it on Facebook (of course). Occasionally, I will head over to the site to brush up on my vocabulary and lately I have been testing my geography knowledge. I even used it to study for the GRE.
The beauty of this site is that it is a win-win and is, oddly enough, brainless. Participants increase their knowledge on subject ranging from vocabulary to geography to art history by playing a game against themselves. While doing so, they are donating 10 grains of rice for every right answer. It is so simple, yet so addicting, that you can easily waste (or contribute) 30 minutes playing and completely forget that you were playing to donate rice.
I can now correctly identify where Tajikistan is on a map 🙂