Programmable credit card sized PIM / organizer?

[Jonathan Busby]

I remember visiting some site on the web, years ago, which had a very active community of software and hardware "hackers", similar to the HP community, which was centered around reverse engineering a programmable credit card sized PIM's / organizer's OS and hardware and writing new assembly level code for the device. The problem is that I can't find said site anymore and I don't remember what the name of the device was. There are probably many devices that might fit the above description, but I'd know the one I'm looking for if I could remember more details, so, I apologize for any vagueness.

My interest is just casual, although I'm a pathological perfectionist so I'd be very happy if I could find out what I can't currently remember regarding the device

Any hints or data would be appreciated,

Jonathan