Website:Tomato is a small, lean and simple replacement firmware for Linksys' WRT54G/GL/GS, Buffalo WHR-G54S/WHR-HP-G54 and other Broadcom-based routers. It features a new easy to use GUI, a new bandwidth usage monitor, more advanced QOS and access restrictions, enables new wireless features such as WDS and wireless client modes, raises the limits on maximum connections for P2P, allows you to run your custom scripts or telnet/ssh in and do all sorts of things like re-program the SES/AOSS button, adds wireless site survey to see your wifi neighbors, and more.
So essentially Tomato is a third party firmware for routers using specifically broadcom cpus. It's basically like DD-WRT, but in my humble opinion better.
Because it has lots of things DD-WRT has, but also has better QOS and bandwidth monitoring features that puts it ahead of DD-WRT.
There are many new variants of tomato firmware forks, the most popular being the following:
Toastman (my personal favourite as he usually takes all the good parts from the other variants and compiles into his)
My Tomato router i'm using is a Linksys E4200 loaded with Toastman's tomato firmware
QOS guide by Toastman himself (very good and detailed read)
Here is an easy setup for Toastman's QOS
Why use Tomato third party firmware rather than the default/stock firmware for routers ?
Usually there is nothing wrong with the default. But 99.9% of the time, tomato will have way way more features compared to the default firmware for your router brand. Especially in regards to QOS setup and bandwidth monitoring which tomato excels at, besides providing other cool features like Client wireless mode, open vpn and more ....