Mobile Chat Kit - UI kits & sample apps for building in-app mobile chat
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
Ambient Design - Ambient is a mobile app design market where you can get unique mobile UI kits.
Landing UI Kit - Handcrafted UI components for landing pages
iOS Design Kit - The newest library of native iOS templates
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.