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

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

CNet

Edimax

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

Gigabyte Technology

GN-WP01GS PCI Wireless Adapter- requires RaLink RT2561/RT61 802.11g

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 nonfree binary firmware making it incompatible with a free system.

Linksys

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

Linksys WUSB54GC USB 2.0 802.11g/b 54Mbps (11 Mbps with b)

Ralink

Conceptronic C54RU- requires proprietary firmware

RaLink RT2561/RT61 802.11g- requires proprietary firmware

Rosewill

Wireless G PCI Card model RNX-G300LX -requires RaLink RT2561/RT61 802.11g

Sagem

X6-76NA that requires the non-free zd1121/zd1211_ub firmware. Also, Sagem XG-76NA cards are also labeled and sold as Sagem XG-762N.

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.