Authman - Cross-Platform 2FA TOTP App
Source-code: https://github.com/simular/authman-app
Ente Auth - Open source 2FA authenticator, with E2EE backups
Protect your accounts with Ente Auth - Free, open source, cross-platform 2FA authenticator, with end-to-end encrypted backups
Yubico Authenticator
Secure your accounts and protect your data with the Yubico Authenticator App. Get authentication seamlessly across all major desktop and mobile platforms.
Quiet - Private messaging. No servers.