Open source self-hosted small business tools

I'll start by listing some of the tools that most if not all of my businesses use:

Nextcloud -- Not only is this a dropbox replacement for file management, it is a platform with many features I recommend like Collabora/ownpad, and Calendar and Contact syncing (I don't use or endorse any Google products, ever). Use this to replace Dropbox, and Google Drive/Calendar/Contacts/Sheets/Docs.

Signal - This is for secure video and voice calls, as well as messaging. You might also check out Wire.com. Use this to replace Skype/WhatsApp/Messenger.

Jitsi - This can be used for group video chats as a replacement for Zoom/appear.in/Skype.

Postfix+gnupg+Enigmail+Thunderbird (K9-Mail on Android) - This is how we use email. For contractors, I ask them to use Tutanota or Protonmail. Use this to replace Gmail/Yahoo/Hotmail. Which tools are your organization using?