Rex 6000 MicroPDA
The Rex 6000 MicroPDA was originally touted as the worlds smallest full function digital assistant. It comes with 2mb of memory to store thousands of contacts and appointments and its the size of a credit card--more specifically a PCMCIA card.
Unfortunately, Xircom, the maker of this, was bought out by Intel and Intel decided not to continue production of it. There is still a large user base and you can still sometimes find them on eBay for anywhere between $60 and $130. I still run one of the last few websites, a discussion forum, at rexforum.com.