The free or open source tools I use in my day job

Current

Proxmox - repurpose old hardware for Lab and testing Virtual Machines
Docker - easy linux image deployment and testing
Portainer - Docker Manager

Uptime Kumar - on Docker via Portainer - important servers and service uptime monitoring
Dashy - on Docker via Portainer - cool shortcuts

Kali Linux Distro install on Proxmox for various security testing and pen testing
hashcat
breachparse

burpsuite

Wazuh VM on Proxmox for SDR and SIEM
OpenVAS (gvm) on Proxmox for pen testing
Netbox VM - Network Documentation
MediaWiki VM - Documentation
phpbb - IT Forum for internal use /discussion

KIX32 - login script and general automation of repetitive tasks, software removal, reg edits, software install, software updates, windows updates, 

Tools I've used but since dumped