This is a great UI kit based on the Android Material UI theme guidelines. If you are designing a new android app this would be great. Source: UI8
Trying to get the design for a web app right is hard work. This design for a todo list app is really good. I particularly like the dark color scheme he is used. Source: Deviant Art
This is a great UI pack for those looking for a flat UI for there apps.
I love the fonts, colours and layouts for this outlook mockup. If only more email clients looked this good. Designer: Frantisek Kusovsky Source: Dribbble
A clean no-nonsense UI with a good colour palette. Designer: Matthew Daniels Source: Design Kindle
This is a great clean UI kit which is available to download from 365psd. Designer: Sebastien Gabriel Source: 365 PSD
This is an excellent free UI for designers which could be used for a range of projects. Designer: Designmodo Source: Designmodo