Docker - Its basically a way to spin up little isolated environments on one machine (similar to a VM). The difference is that the OS of each environment uses the same hardware and kernel as the host machine, so there's no need to simulate either one. Its super fast and makes running a bunch of different services on my home server much easier. Here's a great introductory video and tutorial.

WireGuard - Its an open source VPN that I can self host. The main reason is so that I can access my home server and Umbrel node without having to use Tor.

Jellyfin - An open source media server with a companion app. I have it hosted on a Raspberry Pi 4 sitting near my router.

Before Launcher - Not very as geeky as the other things on my list, but its a minimalistic launcher for Android. Changes the way my phone looks so that its less distracting. It also filters notifications.

NewPipe - An open source YouTube alternative. It still uses YouTube in the backend, but you don't have an account. Its better for privacy in that regard. You still have the capability to do subscriptions and create playlists. You just have to backup everything yourself. It also lets you do background play and downloads.

KeePass - KeyPass is an open source password manager that is locally hosted on your machine. It saves all your logins in an encrypted database file that you can share to other devices if you want. There are a bunch of implementations for IOS, Android, Mac, Linux, and Windows. I prefer KeePassDX as my implementation of choice.

Obsidian - Obsidian is a note taking app like Notion. All your notes are stored as Markdown text files. It lets you link together different notes into a web of knowledge. I use it for personal journaling and my Zettelkasten.

NextCloud - A free and open source alternative to Google Drive, OneDrive, Dropbox, etc. I self host this on the same home server that I use Docker on. It comes with companion apps for almost all platforms.

SyncThing - Another free and open source way to sync files across devices. I use this for Obsidian and to quickly share files between my PC and phone.

F-Droid - An alternative app store for Android devices. It a great way to get applications that are free, open source, and not usually allowed on the Google Play Store. You can get many of the apps I've listed here on this app store and add repositories to expand the selection.