DD-WRT is a Linux-based firmware for wireless routers. It is compatible with several routers, most notably the Linksys WRT54G series (including the WRT54GL and WRT54GS). As with similar projects, DD-WRT is a third-party firmware designed to replace the firmware that ships pre-installed on many commercial routers. This is done for a variety of reasons, including the addition of features which are not typically included in a manufacturer's router firmware.
DD-WRT includes such features as support for the Kai network, daemon-based services, IPv6, Wireless Distribution System, RADIUS, advanced quality of service, radio output power control, overclocking capability, and software support for a Secure Digital card hardware modification.
Buffalo Technology and other companies have shipped routers pre-installed with a customized version of DD-WRT.
Read more about DD-WRT: Version History, Features, Security Issues