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:
There is a semi-official MADWiFi FAQ:
There is a MADWiFi wiki: