OpenWrt

OpenWrt replaces a router's stock manufacturer firmware entirely, unlocking features consumer router firmware typically locks away or never implements at all -- advanced VLAN configuration, VPN server/client setups, ad-blocking at the DNS level, mesh networking, custom firewall scripting, and full package-manager access (opkg) to install additional networking software directly on the router. It supports an enormous range of router models and embedded hardware across many CPU architectures, maintained by a large community that tracks new hardware as it becomes available, though installing it does carry real risk of 'bricking' unsupported or improperly-flashed devices. It's the direct technical foundation several commercial and semi-commercial router firmware products have built on top of, reflecting how thoroughly it's become the reference open embedded-router platform.