If you haven't heard, flat design is pretty trendy right now.
Last September, LayerVault shared their thoughts (and some would say coined the term too) on The Flat Design Era. Since then, many sites and designers have taken to this new trend -- including the folks at Designmodo.
A month ago they released Flat UI, a free and open source user interface kit. If you were paying close attention, you'd know that LayerVault slapped them with a DMCA take-down notice issued to GitHub saying "I am the exclusive rights holder for the artwork contained within Flat UI, Free Web User Interface Kit; These exclusive rights are being violated by material available upon your site at the following URL designmodo.github.com/Flat-UI" -- but that take-down has since been lifted and the air is clear. In the end LayerVault was simply saying that a few of the icons were way too close to their original flat UI designs and had to defend themselves. It happens.
You should check out Flat UI -- even if just to get some ideas on creating your own UI kit.