Free Rice (an academic challenge that donates rice to the UN) http://www.freerice.com
(Sample link) National Geographic for Kids http://www.nationalgeographic.com/kids/
Halloween Hangman (seasonal hangman game) http://www.dedge.com/flash/hangman
Web Sudoku http://view.websudoku.com
Blest Are We (religion-includes self tests) http://blestarewe.com/family
MLA Citation Maker/Oslis http://www.oslis.org/elementary/index.php?page=citeSources