The following are some sites that I have found that provide online games of an educational nature, suitable to the middle school level. I have broken them down by subject for easier navigation. I add new ones as I come across them, so check back often!
Many educational games for every subject matter!
Free Rice This is a vocabulary game, but more importantly, 20 grains of rice are donated through the UN World Food Program for each word you correctly define. Click on "Options" to change the difficulty level, then scroll down the page and click on "Set Options", then click on "Home" to start the game.The Ultimate Word Games Guide