A number of sites offer games that will help you improve your skills while helping others. Here's a list of the games I have found so far. I'll add others as I find them. If you find a game that we could include in this list, please let me know.
Word games and grammar games are the most relevant here. The site also includes art games, geography games and more. For each answer you get write, Free Rice donates 10 grains of rice to the UN World Food Program.
Similar to Free Rice, but the questions seem to be entirely geographic.
More geography, but playing against the clock...
Remember numbers and learn interesting facts, while giving water to those who need it.
You click daily. Sponsors pay for books that go to children around the world. No games, but the cause is a good one.