www.freerice.com
This is pretty entertaining. You get a never ending vocab test and each correct answer earns a donation of a few grains of rice for the needy around the world. The rice is paid for by the advertisers on the page (they have some big names). A pretty good idea overall.
If you get 3 answers correct in a row you are bumped up to the next level of difficulty. If you miss one you drop to the previous level. The highest level is 50 and the FAQ page states it's rare for anyone to get over 48. I've been as high as 45 but I can't stay there for long. I'd bet we have a few here who could hit 48.