FlatDesign is a showcase of some of the best examples of web design using the flat UI style/aesthetic.
The Fresh Exchange - webdesign
Be Visionare is a fantastic example of flat web design with detailed, but not distracting, illustrations.
Buy Flat Web Elements - Modern Design Set by josweb on GraphicRiver. A useful collection of modern flat web elements for your design projects. You’ll find everything from badges to stars.