Get Shit Done - Free Bootstrap 3 UI kit
Mobile Chat Kit - UI kits & sample apps for building in-app mobile chat
NOW UI kit by InVision - A beautiful cross-platform UI kit (PSD & Sketch)
Ambient Design - Ambient is a mobile app design market where you can get unique mobile UI kits.
Spaces by proto.io - A community to showcase your interactive prototypes
iOS Design Kit - The newest library of native iOS templates