Iconfinder is the best icon search engine. There are a lot of high-quality icons available, and you can filter the results to show only those that are licensed for commercial use.
Icon Fever
Icon Fever is our own gallery site of icons. You can browse icons by license to find those that can be used for commercial projects, and you can also browse by tag.
IconsPedia
IconsPedia includes and icon search engine and the archives can be browsed by category.
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