SmallTalk is a minimal messenger using the Matrix protocol.

It has a comparingly tiny app size and focuses on reliability and stability. Its feature set is rather bare-bones:

Login with Matrix ID/Password
Combined Room and DM interface
End to end encryption
Message bubbles, supporting text, replies and edits
Push notifications (DMs always notify, Rooms notify once)
Importing of E2E room keys from Element clients
UnifiedPush
More to come in the future.