Obtainium: https://github.com/ImranR98/Obtainium

Updater for apps released through GitHub, F-Droid, GitLab, etc.

Revanced Manager: https://github.com/revanced/revanced-manager

Patcher for YouTube which adds AdBlock/Sponsorblock/Other various features. Also supports other apps, such as YouTube Music. This is a must if you use YouTube on your phone.

Seal: https://github.com/JunkFood02/Seal

GUI for yt-dlp, which supports so many websites. It can download videos and/or audio, implement sponsorblock to skip intro/outtros, and much more.

xManager: https://github.com/xManager-App/xManager

Installer for modded Spotify. Includes AdBlock, unlimited skips, and the ability to play any track on demand, using a free Spotify account. (The installer itself includes an ad that plays when downloading the patched APK, but this can be disabled in the settings of xManager)

Tachiyomi: https://github.com/tachiyomiorg/tachiyomi

Feature rich manga reader that can pull from various sources. Some people may recommend using one of the various forks.

Adaway: https://github.com/AdAway/AdAway

Non-root/root system-wide AdBlock solution

Yet Another Call Blocker: https://gitlab.com/xynngh/YetAnotherCallBlocker

FOSS & Privacy friendly call blocker that includes support for blacklist/whitelist with wildcard support.

SD Maid SE: https://github.com/d4rken-org/sdmaid-se

Storage utility/manager that's actually good, unlike 99.99% of cleaner apps on the play store. Pro version is free on GitHub, make sure to support the developer if you like the app.

KurobaEx: https://github.com/K1rakishou/Kuroba-Experimental

Feature rich 4chan app

Librera Reader: https://github.com/foobnix/LibreraReader

Feature rich ebook reader. Pro version is free on GitHub, make sure to support the developer if you like the app.