Classy icons crafted according to Microsoft guidelines. Available in PNG and SVG. Free for a link
Feather Icons - Simply beautiful open source icons
Octicons - Your project. GitHub's icons.
IconStore - Free icon packs by first-class designers
Android Icons - Androidicons is a professional, handcrafted icon set with 250 icons in 14 different colors and 5 sizes, including an icon font and all sources.
Flat Icons - 20,000 unique, customizable icons with free lifetime updates
Mononoki Typeface - Font for programming and code review