Systems/Services
Operating Systems
Slackware - the most "Unix-like" Linux distribution.
OpenBSD - multi-platform 4.4BSD-based UNIX-like operating system.
HardenedBSD - HardenedBSD aims to implement innovative exploit mitigation and security solutions.
Kali Linux - Linux distribution used for Penetration Testing, Ethical Hacking and network security assessments.
Parrot Security OS - cyber security GNU/Linux environment.
Backbox Linux - penetration test and security assessment oriented Ubuntu-based Linux distribution.
BlackArch - is an Arch Linux-based penetration testing distribution for penetration testers.
Pentoo - is a security-focused livecd based on Gentoo.
Security Onion - Linux distro for intrusion detection, enterprise security monitoring, and log management.
Tails - is a live system that aims to preserve your privacy and anonymity.
vedetta - OpenBSD router boilerplate.
Qubes OS - is a security-oriented OS that uses Xen-based virtualization.
HTTP(s) Services
Varnish Cache - HTTP accelerator designed for content-heavy dynamic web sites.
Nginx - open source web and reverse proxy server that is similar to Apache, but very light weight.
OpenResty - is a dynamic web platform based on NGINX and LuaJIT.
Tengine - a distribution of Nginx with some advanced features.
Caddy Server - is an open source, HTTP/2-enabled web server with HTTPS by default.
HAProxy - the reliable, high performance TCP/HTTP load balancer.
DNS Services
Unbound - validating, recursive, and caching DNS resolver (with TLS).
Knot Resolver - caching full resolver implementation, including both a resolver library and a daemon.
PowerDNS - is an open source authoritative DNS server, written in C++ and licensed under the GPL.
Other Services
3proxy - tiny free proxy server.
Security/hardening
Emerald Onion - is a 501(c)(3) nonprofit organization and transit internet service provider (ISP).
pi-hole - the Pi-hole® is a DNS sinkhole that protects your devices from unwanted content.
maltrail - malicious traffic detection system.
security_monkey - monitors AWS, GCP, OpenStack, and GitHub orgs for assets and their changes over time.
firecracker - secure and fast microVMs for serverless computing.
streisand - sets up a new server running your choice of WireGuard, OpenSSH, OpenVPN, and more.