I've mentioned Burnstudio's Audiotool before, but it's worth a second mention. It's an amazing on-line music creation studio. You can drag and hook up various expensive sound modules, effects pedals and more. Once you've got a nice rhythm flowing you can play/pause using a switch hooked up to the SPACE BAR. Alternatively, you can use left-click to switch some elements on and off that with pre-planning can be very effective.
Alternatively, if you have accurate point, click and dragging skills you'll have massive control over creating some fantastic sounding music.