MADWiFi, or the Multimode Atheros Driver for Wireless Fidelity, is a driver for Linux which supports most current Atheros chipsets (such as 5210, 5211, 5212, ...).
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 project website:
The wiki on that site has lots of useful information, such as installation and configuration recipes, an FAQ and other stuff: