With no introduction, you need to give this a try: http://www.incredibox.com/v3/
With an introduction: this website lets you drag and drop beats, sounds, melodies, and small voice parts, 7 at a time, into the window for cartoon beatboxers to sing for you. It’s hypnotizing, well-put-together, includes a secret video, and has a shuffle mode you can just leave on (which I am).
Give it a shot. For background music, it almost can’t be beat.