Quagga!

The main product of MikroTik is a Linux-based operating system known as MikroTik RouterOS. Installed on a personal computer or server computer, it turns the computer into a network router, implementing features such as firewall rules, virtual private network (VPN) server and client, bandwidth shaping and quality of service, wireless access point functions and other commonly used features for routing and interconnecting networks. The system is also able to serve as a captive-portal-based hotspot system. The operating system is licensed in increasing service levels, each releasing more of the available RouterOS features. A Windows software application called Winbox provides a graphical user interface for the RouterOS configuration and monitoring, but the software also allows connections via FTP, telnet, and secure shell (SSH). An application programming is available for direct access from custom applications for management and monitoring. »

BIRD is an open source implementation of a TCP/IP routing daemon for Unix like systems. It supports both IPv4 and IPv6, multiple routing tables, and a BGP, RIP and OSPF routing protocols, as well as statically defined routes.

freeRouter is a routerOS process.it means freeRouter speaks routing protocols, and reencapsulates packets on interfaces.(a huge list of encapsulation and routing test cases can be found under self-test results) since it is an unprivilegized process, it receives and sends packets through sockets.there are external, privileged processes that place traffic to these sockets.(it means that internet can be used as backplane for router processes)the command line tries to mimic the industry standards with one exception: no global routing table: every routed interface must be in a virtual routing table.(positive side effect: there are no vrf-awareness questions)complex self-tests: connectivity tests executed on a network of virtual routers and these test results are compared to predefined values. »

exabgp - The BGP swiss army knife of networking.

Junos OS (more formally Juniper Network Operating System) is the FreeBSD-based operating system used in Juniper Networks hardware routers.It is an operating system that is used in Juniper's routing, switching and security devices. Juniper offers a Software Development Kit (SDK) to partners and customers to allow additional customization. The biggest competitor of Junos is Cisco Systems' IOS.Junos OS was formerly branded as Juniper Junos, and is commonly referred to as simply Junos, though this is a general brand name of Juniper Networks, including other product lines such as Junos Fusion. »

XORP is an Internet Protocol routing software suite. It provides a fully featured platform that implements IPv4 and IPv6 routing protocols and a unified platform to configure them. It is the only open source platform to offer integrated multicast capability. XORP's modular architecture allows rapid introduction of new protocols, features and functionality, including support for custom hardware and software forwarding.