MADWiFi, or the Multimode Atheros Driver for Wireless Fidelity, is a driver for Linux which supports Atheros 5210, 5211, and 5212 chipsets.
It works for 802.11a, 802.11b, and 802.11g, and is mostly OpenSource; the parts that are not are required by law due to FCC regulations. The driver was ported from FreeBSD; that OS has native support for these devices. It also supports infrastructure (also called "access point", "Host AP", or "master") mode, WDS (wireless distribution system) support, and many other advanced features.
MADWiFi SourceForge project page:
http://sourceforge.net/projects/madwifi
There is a semi-official MADWiFi FAQ:
http://www.mattfoster.clara.co.uk/madwifi-faq.htm
There is a MADWiFi wiki:
http://madwifiwiki.thewebhost.de/wiki/FrontPage
[Note that the wiki seems to be down at the time of writing - KeeganQuinn, 20040108]