Trust me when I say this, Ableton's Little Drummer Toy sequencer is addictive. The music is simply hypnotic and it sucks you in with it's steady melody and mesmerizing visuals.
The unique interface pairs you with other users from around the world to allow you to collaborate on a project. Personally, I've found that most users don't really want to collaborate and leave the site once they realize there is another person who can add their input. Nevertheless, it's a really fun way to experiment with music.
To play with the Little Drummer Toy sequencer from Ableton, click here. Also, you can try the HTML-909 in-browser sequencer to get even more creative with your own productions.