I’ve been using an old Linksys WRT-54G (version 2.0) for years, while Linksys released a few firmware updates over the years, none of them really added any new functionality. Thankfully, they had released the source code to the open-source community and several excellent releases have been made available.
Additions such as real-time monitoring/graphing, boosting wireless transmitter power, QoS (Quality of Service) and WOL (Wake-on-LAN) have been added making the router and network much more useful and valuable.
The most prominent versions are:
NOTE: There are several branches on both of these, you can read more at their respective websites.
I was easily able to update from the Linksys firmware (4.21.1) to Tomato 1.25 in only a few minutes, it was even capable of saving my existing configuration making the transition that much easier!