Do-It-Yourself Internet Service Providers

• bsdrp: Appliance-style OS to build a router, free/nanoBSD-based. Feature list here.
• VyOS: Fork of Vyatta. Feature list here.
• OpenWRT: Famous opensource firmware for embedded devices.
• pfSense: FreeBSD-based firewall OS with lots of features.

See also this list on Wikipedia.

• Quagga: Multiprotocol routing daemon, fork of the venerable zebra. Supports RIPv1, RIPv2, RIPng, OSPFv2, OSPFv3, IS-IS, BGP-4. List of supported RFCs here
• Bird: Very efficient bgp/rip/ospf/bfd routing daemon. Supports for multiple routing tables.
• Openbgpd: Secure bgp routing daemon. OpenBSD has also basic MPLS support.
• Babel: loop-avoiding distance-vector routing protocol, great for mesh wireless networks. FAQ here.
• BATMAN: B.A.T.M.A.N. (better approach to mobile ad-hoc networking) is a routing protocol for multi-hop ad-hoc mesh networks.
• XORP: supports OSPF, BGP, RIP, PIM, IGMP, OLSR. Exists also as appliance.