ToneMatrix, a labs experiment from one of the guys behind the Hobnox Audio Tool is... mesmirizing. The concept is simple - there's a grid, and each block in the grid represents a specific tone. An invisible playhead constantly loops across the grid and triggers the tones. You toggle the blocks 'on' and 'off' with a mouse click.
Create your own patterns and fine-tune your melody, or draw pictures and see what they sound like - ToneMatrix can keep you occupied for quite a while. Check it out!