ChatSecure is a messaging application for iOS which allows OTR and OMEMO encryption for the XMPP protocol. Developed by Chris Ballinger and The Guardian Project
License: Open Source
Threema is a paid messaging app that emphasizes privacy and anonymity, requiring no phone number for registration, while ChatSecure is an open-source app that supports multiple protocols and integrates with Tor for enhanced anonymity. Both apps provide strong encryption, but Threema is more user-friendly, whereas ChatSecure may appeal to technically inclined users.