Linux
"Understanding TCP/IP Network Stack & Writing Network Apps" - a good article to get a conceptual view. 2012 (exact publishing date is not clear)
KVM
http://www.linux-kvm.org/page/Networking_Performance
P4
Programming language. "P4 programs specify how a switch processes packets."
Links: home
NPL
High-level network programming language. Backed by Broadcom.
Links: home
Open Network Install Environment (ONIE)
Abstraction layer between bare metal network switch (hardware) and operating system. It is like a BIOS for a network switch.
Links: home
Open Network Linux
Linux distribution for "bare metal" switches, that is, network forwarding devices built from commodity components.
Links: home
OpenSwitch Network Operating System (NOS)
Community-based, open source, full-featured network operating system
Links: home, HP press release
SONiC
Open source project for network routers and switches. Backed by Microsoft
Links: home
Metaswitch
Links: L2, L3, MPLS, VPN networking software
Atomic
OS for Linux Docker Kubernetes (LDK) application stack
Links: home
Yocto Project
" An open source collaboration project that provides templates, tools and methods to help you create custom Linux-based systems for embedded products regardless of the hardware architecture"
Links: home
OpenDataPlane
Open-source, cross-platform set of application programming interfaces (APIs) for the networking data plane.
Links: Home
Open-NFP
"community-driven organization that enables open and collaborative research in the area of network function processing in server networking hardware"
FRR
"FRRouting (FRR) is an IP routing protocol suite for Linux and Unix platforms which includes protocol daemons for BGP, IS-IS, LDP, OSPF, PIM, and RIP."
GoBGP
About: 'GoBGP is an open source BGP implementation designed from scratch for modern environment and implemented in a modern programming language, the Go Programming Language.'
Links: GitHub