http://findicons.com/
https://www.iconfinder.com/
http://www.flaticon.com/
http://www.iconarchive.com/
http://www.playpcesor.com/2014/11/Flatty-Shadow-Flat-icon-download-free.html
http://www.premiumpixels.com/page/1/?s=icon
http://www.bestpsdfreebies.com/category-freebie/icons/
https://github.com/google/material-design-icons/blob/master/README.md
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原文：http://blog.csdn.net/wu2304211/article/details/47386645