I found a great site last week that goes along with the composer study/music history theme, but this one is geared toward younger ears. Classics for Kids is a website dedicated to introducing children to classical music. I love the way it is arranged! You can easily listen by composer, by period, or just chronologically work your way through the list. There's even a handy dandy timeline if you're into that sort of thing (we are).
There's much more to explore there, too, with printable activities, online games, and info on all the orchestral instruments. You can even learn about musical careers or consult the musical dictionary, and it's all in a kid-friendly environment.
Aside from their own collection, Classics for Kids has a link to Naxos.com, a web radio environment where you can search content and play your selections. At Naxos, you can listen to 20% of any piece without paying for the subscription, however, an annual subscription is a bargain at $9.95 for FM quality or $19.95 for CD quality.