Junos OS is described as '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'. There are five alternatives to Junos OS for a variety of platforms, including Self-Hosted solutions, Linux, Online / Web-based, BSD and GNS3. The best alternative is MikroTik RouterOS. It's not free, so if you're looking for a free alternative, you could try Quagga or securityrouter.org. Other great apps like Junos OS are VyOS (Paid, Open Source) and Cisco IOS (Paid).
Quagga is a network routing software suite providing implementations of Open Shortest Path First (OSPF), Routing Information Protocol (RIP), Border Gateway Protocol (BGP) and IS-IS for Unix-like platforms, particularly Linux, Solaris, FreeBSD and NetBSD.
The securityrouter.org project is a network operating system and software distribution based on OpenBSD, featuring a single, revision-managed, clear-text configuration file with soft re-configuration, editable from CLI and web interface.