I've written a waterfall display program for the Arduino microcontroller and a 128x128 pixel display.
You can build it yourself for about $80, or make a smaller version using smaller parts and put it in a box for use with your KX1, K2, or KX3.
For those with iPods or iPads, there is a set of apps written by Luca Facchinetti that include waterfall displays, decoding and sending of PSK31, RTTY, and QPSK. I have found them useful in preparing my emergency response kit. They are named "Multimode" and "IKpsk".
This project doesn't do decoding, just display. But it's a good DIY project to learn about how these thigns work, and you can extend it yourself. I've provided a few ideas for more work that anyone can do.