PBS
Pbs: Elements of Music: Exploring the Composer's Tools
This online lesson will help students understand how composers write music by engaging in a project which culminates in their own compositions.
Other
Chord Studio
Have fun composing your own music with this easy to use online tool. Select the chords you want your song to have and choose the various instrument styles. You can sign up for a free account so you can save your songs and edit them later.