What are your favorite FOSS apps?

For iOS:

Fully FOSS:

• Organic Maps - Offline Maps
• Yattee - YouTube Frontend
• BookPlayer - Audiobook Reader
• NetNewsWire - RSS Reader
• Signal - Encrypted Messaging
• Element - Matrix Client
• OrBot - Tor VPN Alternative
• Swiftfin - Jellyfin Client
• VLC - Media Player
• Metapho - Remove EXIF Data
• Privacy Blur - Blurring Photos
• Raivo - 2FA
• Firefox Focus - Web Browser
• Mastodon - Official Mastodon Client
• PixelFed - Official PixelFed Client
• Sync - Reddit App (RIP?)
• Mlem - Lemmy App (currently testing)
• KDE Connect - Airdrop Alternative
• Berty - Encrypted Messenger with Offline Capabilities

FOSS, but with a paid tier:

• KeePassium: Offline Password Manager
• Slide for Reddit - Reddit App (RIP)
• AdGuard - Content Blocker
• Bitwarden - Password Manager
• Joplin - Note App
• Podverse - Podcast App
• NextDNS - DNS Filtering
• Invoice Ninja - Invoicing Software
• SimpleLogin - Email Aliasing
• ProtonMail - Encrypted Email
• ProtonCalendar - Encrypted Calendar
• Proton Drive - Encrypted Cloud Storage (Testflight is Full)
• ProtonVPN - VPN
• Standard Notes - Encrypted Notes
• Notesnook - Encrypted Notes
• Tutanota - Encrypted Email & Calendar

Paid Only, but Open Source:

• Cryptomator - File Encryption
• ente Photos - Encrypted Photo Backup
• Mullvad VPN - VPN
• IVPN - VPN

Privacy Friendly, but not FOSS apps:

• QuickScan - Document Scanner
• Hello Weather - Weather App
• MySudo - Phone Number Aliasing
• Mobius Sync - Syncthing Client
• Privacy.com - Virtual Credit Cards

Edit: Added ente Photos, Mullvad, IVPN, Tutanota, Standard Notes, Notesnook, and Berty

I have neck issues and emulating my phone on screen is a must! I cant get enough of 'guiscrcpy' Open source Android screen mirroring! Multiple phone support, works perfect! https://guiscrcpy.srev.in/

Just mentioning the tools I had not seen on other posts:

Selfhosting

• https://github.com/wallabag/wallabag - Instapaper/Pocket alternative. Has mobile clients and hosted version as well
• https://github.com/paperless-ngx/paperless-ngx - Great document management system, has multiple apps by third party developers
• https://grocy.info/ - ERP for your household (food inventory, grocery list, chores) - there are multiple open source clients for it as well

Android

• https://f-droid.org/en/packages/net.osmand.plus/ - Routing based on OpenStreetmaps
• https://f-droid.org/en/packages/org.oxycblt.auxio/ - Audio player
• https://f-droid.org/en/packages/ws.xsoh.etar/ - Calendar
• https://ntfy.sh/ - Pushbullet alternative, can be selfhosted as well. Works as a - https://unifiedpush.org/users/apps/ provider.

Darktable is a great photo editor (similar to Lightroom).

If we're talking about mobile apps only then ntfy, Fluffy Chat, KDE Connect, Nextcloud, Tuta, Protonmail, Session, Conversations, Organic Maps, Fedilab, Mull, DAVx and KeePassDX.