Go to ClubCreate.com and you’ll find a collection of web apps to compose your heart away. I happened to play with “The Music Studio“. There are more web app options to choose from, so after reading you can check them out here.
About ClubCreate (via ClubCreate.com):

ClubCreate stands at the intersection of music, technology and culture providing free, web-based music creation tools for the masses. ClubCreate breaks down the barriers to music creation providing Brands with unique ways to engage new customers. We have worked with top global brands such as Microsoft, MOG, Myspace and Vodafone to create these unique experiences that help promote brand awareness via music.
I’ll talk about the good first.
There are a lot of loops and options. I could take a loop and add reverb, delay, reverse, flanger, phaser, 3 band EQ, normalize, and distortion if I wanted to. You can also edit pan and volume by inserting nodes and moving them around. I also had the ability to solo and mute.
Working within the app is really easy, just drag and drop. You can’t cut a loop (I couldn’t find an option), but you can add or delete the loop by clicking on a ‘measure’ (simply put).
The Music Studio is actually a lot of fun, but I had some odd experiences. For example, I’d add the same loop a few times, then delete a couple of them and the specific track would no longer work. I don’t know why, but after clicking a whole bunch of times, it eventually worked again. My biggest frustration, though, was when I saved the project and came back to it. The whole project was a mess. Everything was in it’s place, but a lot of the tracks, more specifically tracks with effects, did not have the appropriate sounds. It was really annoying and after awhile I finally (or hopefully) got it back to the way it was supposed to sound. You’ll notice quickly what I’m talking about when you watch the video I created compared to the widget they offer to play your track in. You’ll notice that a lot of the sounds are just straight up wrong. I found the program unreliable.
You don’t necessarily have to be very creative to make something cool. That said, if you want you can get creative with what you have available. I found that with the limitations I was still able to change the project to my liking.
Check out the video to see/hear my demo ‘composition’, then listen to the widget track. You’ll have to skip ahead in the widget track because I had a few measures in the beginning I couldn’t get rid of. You’ll quickly notice the difference.
Anybody up for throwin’ a rhyme on top of the beat?
It’d be cool if someone rapped over my mix. Surprise me! Let me know and I’ll add it to this post.