General Knowledge:

http://www.millionairetv.com/    Who Wants to Be a Millionaire

Geography:

http://www.eduplace.com/geonet/

http://www.funbrain.com/where/

http://www.lizardpoint.com/fun/geoquiz/

http://www.nationalgeographic.com/geobee/

Science:

http://education.jlab.org/indexpages/elementgames.php

http://www.socialstudiesforkids.com/subjects/funandgames.htm

http://www.engagingscience.org/games/