Called Tomato, it is the name of a firmware for wireless routers, including the popular WRT54G family. Tomato is designed to be flashed onto your compatible wireless router to replace the stock firmware loaded by the vendor.
Like DD-WRT, the Tomato firmware can be installed on a variety of wireless routers. Tomato supports the Linksys WRT54G family, which also includes the WRT54GS, WRT54GL and WRTSL54GS. Note that Tomato will not work with WRT54G routers later than version 4, since these do not run Linux.
Unlike DD-WRT, which is available in numerous versions for different routers and with different feature sets, there is only one primary version of Tomato. There is also one alternate version compiled specifically for the Buffalo WHR-G125 router. [Source: Adapted from Wi-FiPlanet.com]