Here is a completely free set that includes 880 icons for user interface design. This set includes 6 categories to choose from (education, summer, construction, food & beverage, medical, electronic devices), and for each of these categories, it provides 4 different styles (Line, Glyph, Flat & Filled Line).
The icons are designed to fit within a 32x32px box. It’s a great choice if you’re looking for something different from the usual sets, which can be used for interface design, but also for illustrations, printable products, and more.
Preview of the “summer” category: