Mobile Chat Kit - UI kits & sample apps for building in-app mobile chat
SendBird - SendBird is messaging-as-a-service.
Ambient Design - Ambient is a mobile app design market where you can get unique mobile UI kits.
Chat by Stream - Build and ship in app messaging in a few hours
iOS Design Kit - The newest library of native iOS templates
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.