Technically, these are CardBus, not PCMCIA. PCMCIA is the same form factor but is much older/slower (CardBus is PCI, PCMCIA is ISA). You will likely not find any PCMCIA cards useful for post-Pentium III systems.
I've used the following CardBus cards in various scenarios:
* USB 2.0 adapters for laptops which only have USB 1.1 (or just to add more USB ports)
* SD Card adapters
* CompactFlash adapters (although that's hardly useful since most of them are dreadfully slow)
* Wireless adapters for laptops that have no built-in Wifi or to get 802.11g over 802.11b
There also exist Firewire and Ethernet CardBus devices, although I have never needed such a card personally.
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