Asus WL-167G USB wireless adaptor -- It uses the rt73 chipset and requires binary firmware to operate.

Broadcom

These cards are not supported, because of their proprietary firmware requirement.

Edimax

These two cards, EW-7128g, EW-7108PCg are not free because they use the RT61 chipset which requires proprietary firmware.

HAWKING HWC54G Wireless-G Cardbus Card -- It was supposed to use the RT2500 chipset, but turned out to require the acx driver. While the driver is free it requires binary firmware making it incompatible with a free system.

Linksys WMP54G Ver 4.1 PCI Wireless card -- It requires proprietary firmware.

Not supported--a straightforward job of reverse engineering is needed:

atmel USB B   "firmware free"
symbol PCMCIA B "firmware free"
[Info from Theo de Raadt]

Not supported, and it would be very hard to do so:

TI, Intel, Prism GT
[Info from Theo de Raadt]
The Intel chips appear to use a special type of computer that has never been publicly announced.