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.

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