I once (mostly) fixed and circuit bent a previously broken Casio PT-1 I got from ebay, which I got mostly to see if I could fix it in the first place. it has 2 added potentiometer controls, one which controls pitch and the other modulates the sound in some way. the switch changes it from sounding normal to sounding like a zx spectrum beeper heh, I think it's pretty neat. I should get some footage of it but I don't really have that kind of setup for video atm.
Also, I once picked up a yamaha pss 6 keyboard for circuit bending as it's purely digital which means there's a greater chance of getting some crazy glitchy stuff from it. I haven't done any circuit bending to it yet, but I did once try running it on low voltage and it already sounded crazy just by doing that!