The first is Plink, a multiplayer toy where players can hop into rooms and experiment with different sounds. You interact with Plink by clicking and dragging your mouse—very simple and intuitive. Probably best experienced than talked about, but the video above should give you an idea of what Plink is like in action.
I'm always pleasantly surprised at how communication arises from low-bandwidth interaction—Journey is a good example—and Plink is no exception. If you nab a room with a few people, you'll silently start coordinating in an attempt to create delicious harmony. If you want to create magic in private, the option to create your own special room is also available.
Regardless, you might smile at what you and others accomplish together.
The second is Tonecraft, another neat toy that has players composing Minecraft-like blocks on a grid, each color being a different instrument. The X and Y axis represent pitch and timing, respectively.
Similarly to Plink, the pleasure is in accidentally stumbling on something that sounds good and just going with it. You'll make some wicked sounds, and also create something visually appealing on your grid canvas, too.
Check them out! If you have Chrome, that is—both are HTML-5 applications.