[http://acx100.sourceforge.net acx100] was a project started by Andreas Mohr to create a free open source driver for acx100 chipsets from Texas instruments. Later produced acx111 chipsets are also covered by this work. At the current point the driver is working quite reliable and is ported to the mac80211 WLAN stack by a guy named Jeff. These chipsets can be found in many devices from PCI over PCMCIA to USB.
acx100: this driver should pretty much work with mac80211 acx100usb: this driver does not work yet acx111: this driver should pretty much work with mac80211
A complete list of devices known to work with acx1xx can he found in [http://acx100.sourceforge.net/wiki/Device_list the acx100 wiki]
not working yet
You need to get a firmware to actually use the device. The [http://acx100.sourceforge.net/wiki/Firmware wiki] should give you a good point to start on which firmware to choose and where to get it.