This is our FLOSS apps recommendation for Android users who use F-Droid alternative play store. You will find many of these apps similar to what we Ubuntu users already benefit from for years. Enjoy!
Subscribe to UbuntuBuzz Telegram Channel to get article updates.
Web browser:
In F-Droid you do not find Firefox. We recommend Fennec, it is the Firefox Mobile for Android with ability to use Firefox add-ons and got proprietary software stuffs removed.
File Manager:
We recommend File Manager by axet described as the File Manager with root browser as it is a fast and full featured Android file manager which is actively developed, than can replace the proprietary built-in one.
NewPipe, Your Tube App:
We also recommend NewPipe, a free and fast alternative that you might say better than official YouTube app. It features search, download, playlist, shuffle and repeat, and queue with privacy (meaning, without Google recording your life). Using this, you can play videos online and offline too. You can store the files in MP4, M4A, WEBM, and OPUS format.
Video player:
VLC is everyone's tool to watch movies, listen to musics, and play any of these kinds. It features playlist, repeat and shuffle, and quick gestures. It is able to play anything including MP4 and MP3 as well as WEBM and OGG.
Games:
Games are available in F-Droid and we recommend Apple Flinger: it is an Angry Birds-like game with many levels and challenges to play with one or two players. You use tap and swipe to shoot the apple. Play this with your kids!
Document Viewer:
We recommend Document Viewer: it is just works PDF reader for Android. It presents your books in a shelf and simply tap to read each one without further configurations needed. It works with multiple formats including pdf, djvu, epub, and cbz.
Email App:
We recommend K-9 Mail, with this great email client, you can access Gmail, Yahoo or any other email services right within your phone and read emails offline. You can read your inbox, send emails, join email groups, search and archive mails, even further use encryption with it.
Social media:
Communication:
There are multiple ways to communicate online at these modern era. F-Droid provide all kinds of them including the popular ones you might already use. Below we listed cross platform choices.
Telegram FOSS: fast messenger your family and friends can use.
Element: all in one messenger, based on Matrix tech, your family can use.
Conversations: XMPP messenger that is very user friendly and recommended by many.
Jami: modern peer-to-peer messenger part of GNU OS with video call.
Utilities:
DiskUsage: a utility everyone should have, it is your disk usage analyzer for Android. This is similar to Disk Usage Utility (Baobab) on your Ubuntu. This scans your storages to present you ranking of folders which use most free space. This helps a lot to maintain your filesystem off of unused folders and files. 
KDE Connect: for Kubuntu users, this app pairs with KDE Connect preinstalled on your computer to help you transfer files, remote controlling, and more. For example, you can further use your phone as remote of your presentation in front of your colleagues.