The best alternatives to SmartIcons are Ego Icons, Basicons, and Swifticons. If these 3 options don't work for you, we've listed a few more alternatives below.
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Basicons is a curation of basic icons for product design and developement. Designed with care & precision. Updated & refined weekly. Basicons is designed on a 24x24 grid, with the strokes intact for high customization.