This site has a section on teaching about the Classical age, as well as a section that teaches about all the instruments in the orchestra.  It also has a monthly featured composer.

Classics for kids:  http://www.classicsforkids.com/

This site has a timeline of classical composers. http://www.danmansmusic.com/composers.htm

If you are trying to develop a music appreciation class for high school, the Oklahoma Homeschool site has one planned for you: http://www.oklahomahomeschool.com/musicApprHSU.html

This site teaches about the different eras of music, over seventy composers, and has a musical glossary.

Essentials of Music: http://www.essentialsofmusic.com/

This site has downloadable teaching kits about famous composers. (You must click the teacher's section.)

Arts Alive:  http://www.artsalive.ca/en/

Find free composers pictures to make flashcards or use in notebooking here: http://www.montessorimaterials.org/music/Composers.pdf

Online adventure for learning about the orchestra:  The Weill Music Institute at Carnegie Hall: http://www.carnegiehall.org/article/explore_and_learn/ovr_weill_music_inst.html

A guide to the orchestra can be found here: (from The British Broadcasting Company): http://www.bbc.co.uk/orchestras/guide/

The Dallas Symphony Orchestra for kids:  http://www.dsokids.com/2001/rooms/musicroom.asp

Creating Music.com allows your child to compose his own masterpieces.  http://www.creatingmusic.com/

The San Fransisco Symphony Orchestra also offers a site for kids: http://www.sfskids.org/templates/home.asp?pageid=1