Mobile Chat Kit - UI kits & sample apps for building in-app mobile chat
iOS Design Kit - The newest library of native iOS templates
Ambient Design - Ambient is a mobile app design market where you can get unique mobile UI kits.
Sketch Repo - Collection of resources for anyone who uses Sketch
SketchApp.rocks - Index of all awesome things related to Sketch.
Chat by Stream - Build and ship in app messaging in a few hours