Aviary, creators of the Aviary online image editor, have just released an online music maker called Roc, using tons of free and Creative Commons-licensed instruments, and an interface that doesn't require any music theory knowledge to work with.
Click on the image for a closer look.
Roc's interface actually looks quite a bit like GarageBand and other similar audio editors. Instead of tracks, though, each row in the grid represents a note on your chosen instrument. The columns represent beats, and the bubbles within the grid represent whether a note is played or not (see the above picture for an example of a simple string of notes). The interface is completely point-and-click, so you just click on a bubble to play that note at that point in the tune. When you're finished, you can download your track as an MP3, to further edit in your editor of choice or keep it as a finished song. They'll also be integrating their own Myna audio editor in the future, so you can merge your tracks and edit a full song in your browser instead of using an external program.
What's especially cool about Roc is that all of the instruments and sounds available have been released under a Creative Commons - Attribution 2.5 Generic license, meaning that you are free to distribute music you make with Roc for any purpose, commercial or non-commercial, as long as you attribute them as creators of the sounds. They plan on adding more instruments and sound groups as time goes on, as well as other features (some of which are available now as beta features), so this is just the beginning. Hit the link to check it out, and be sure to check out their blog post about Roc's release for more information.