OpenWrt
A Linux distribution for routers and other network gear. It emerged in 2004 after Linksys was compelled to release the source for the WRT54G under the GPL — the whole project is a direct consequence of licence terms.
The difference from vendor firmware is that you get a real Linux system: packages install with opkg, configuration lives in text files under /etc/config, and the LuCI web interface is optional. That makes it possible to run WireGuard, ad blocking, VLANs, QoS, monitoring or a small web server on the router. The strongest argument is security: vendors stop shipping updates after a year or two while OpenWrt continues, rescuing perfectly good hardware from becoming a liability. It supports thousands of models with varying driver completeness — check the hardware table first. The project forked into LEDE in 2016 and remerged in 2018.