I remember when I used to have the old, larger style Playstation someone showed me this cool trick. Basically there was a button combination that was pressed either when there was no disc in the system or there was a music cd in it (I can't remember which). The result was colorful visualizations that came up on the screen sort of like you'd see in Windows Media Player that you could manipulate with the controller.
I want to say this could only be done on the later models (right before the PSOne was released) but I'm not sure on that.
Does anyone remember how exactly this is done? I no longer have a Playstation to try it out with.