Music Games Online !

http://www.themonkeyzone.com/lessons/music/

A source of online music games to reinforce the following concepts: music theory (notation, note value), music alphabet, the piano keyboard, and more !

http://www.emusictheory.com/practice.html

http://www.musictechteacher.com/music_quizzes/music_quizzes.htm

http://www.flashmusicgames.com/theory/index.html

this one is a little more elementary, but looks fun to try :)
http://www.makingmusicfun.net/htm/arcade.htm

A FANTASTIC source of Music Activities! Review fundamental concepts with these fun worksheets!
http://www.funandlearnmusic.com/free-music-worksheets/ 

Great source for Music Worksheets & Teacher Resources
http://susanparadis.wordpress.com

Excellent online source for music worksheets for theory review!
http://www.opusmusicworksheets.com

Variety of Worksheets for Music Theory
http://www.gmajormusictheory.org/Fundamentals/workbooks.html

Interesting Site re: the Mysteries of Music & Math
http://www.freeplay.com/Teaching/tuning_mmm.htm