I'm unsure if you're referring to open-source software or hardware, but for software pfSense [1] and OpenWrt [2] are both popular open-source routers.

[1] https://www.pfsense.org/ [2] https://openwrt.org/

The Turris Omnia is an OpenWRT based open source option.

Any running Linux kernel is a router. You just have to know how to configure it. Of course, you might be expecting a lot more than just a router, ie. DHCP, DNS, traffic shaping etc. It's all available in most distros.

Buy a small PC with lots of network ports and install VyOs [1] on it. If I recall correctly, Ubiquiti's EdgeOS is based on VyOs.

1. https://vyos.io/

pfsense and opnsense are BSD based routers.

openwrt is based on Linux