I love Pandora.com, and they have some great stations for children's music. The kids love the music and I don't get sick of the same songs over and over again. It has also introduced our family to some kids' music that we never would have known about.
Go to pandora.com and click on genre stations. Select "Family". There you will find Toddler radio, Lullabye Radio, and Children's Folk Songs. All great stations.
I haven't checked out the "tween" station, but it is probably pretty good as well.