Capers was proposed as a whole replacement OS for Palm PDA devices a long time ago. There are a series of pages under this one that have applets from Capers available for download if you want to play about with them. They only work on pre-OS5 Palm OS devices, so don’t try them on anything like a Tungsten, you’ll be disappointed. Also they mainly work with MIDI over serial.
I a bunch of downloadable PDF versions of the original Capers site, which is now long gone. Click on any of the links below for more.
Also you should take a look at the Palmorama library to see a load more older palm stuff.