Best music theory resource on the internet. Learn and practice the basics of reading sheet music with free lessons. There are also fret-board and keyboard visualizers and quizzes.
This website allows you to produce random rhythms with measure, time signature, and difficulty level variables. I project these on a board in my classroom to practice rhythm labeling.
Web Comic with many musical jokes and band references
Free Rhythm worksheets at many different levels.
Great rhythm generator tool from http://www.practicesightreading.com/. I project this on the board and have students label the rhythms.