MemPlug is a product by Portable Information Technology that is a series of SpringBoards for the HandSpring Visor. The first two, at $50 a piece, can allow your Visor to access CompactFlash and SmartMedia memory cards, two different proprietary formats commonly used to store data from digital cameras and such. Palm has some models which also can run programs from SmartMedia, so presumably armed with the SmartMedia MemPlug Springboard you could exchange programs and such with a Palm Pilot device. Next month they're unveiling a third model which can read the Sony Memory Stick, a third proprietary format. So, armed with an array of these devices, you could use any number of memory cards on your Visor. And to think this little $150 device of mine can do so much...
Also, the other day Panasonic, makers of the Secure Digital memory card (a form of SmartMedia, I believe) announced 256MB and 512MB cards. I know that the Nintendo GameCube uses a proprietary 8MB memory card but they will also unveil an adaptor to use 64MB SD cards from Panasonic (due to an agreement between the two companies). I wonder - can you use the 512MB card on a Game Cube? If you can, then one memory card would do you for life. Better yet - I wonder if you could then pop the same memory card (be it 512MB or 64MB) into your Visor and manage/hack your saved games on your Visor/Palm Pilot - that would be too cool.