I wanted to start this discussion to see what other MSP's are using regarding free open source software. I'll start...

PopOS and Linux Mint - Desktop OS
Firefox - Web Browser
LibreOffice - Document Creation
DrawIO - Diagrams
Pixel 4a with LineageOS - For Mobile Phone
Nextcloud - File Syncing and Sharing, Calendar, Contacts, Talk
TacticalRMM - Remote Management and Monitoring for clients
Meshcentral - Remote Desktop Access
HestiaCP - Web/Mail/DNS/DB Hosting - Also used for DynamicDNS
Rainloop Webmail - For Email
Vaultwarden - Internal Password Management
Proxmox - Hypervisor
Proxmox Backup Server - For Backing up Virtual Machines
Proxmox Mail Gateway - For Client Onsite Exchange server filtering
Dokuwiki - Internal Documentation
pfSense - Firewall
Ubiquiti Unifi - For Network Management - Not really opensource but still self hosting
Ubiquiti UISP - For Firewall/End Device Management
FusionPBX - To Manage VOIP Phones
Zabbix - Network and Device Monitoring and alerting
GrayLog - Central Logging
ITFlow - Invoices, Accounting, Ticketing, Client Documentation, Mail Campaigns
There Probably more..