Free SVG and PNG Icons Easily Customizable For Games
Some other icon sets (probably appeared on HN before, don't know where I got them from):<p>flags: 
<a href="https://www.gosquared.com/resources/flag-icons" rel="nofollow">https://www.gosquared.com/resources/flag-icons</a><p>user interface: 
<a href="https://www.gosquared.com/resources/165-vector-icons" rel="nofollow">https://www.gosquared.com/resources/165-vector-icons</a> and 
<a href="http://adamwhitcroft.com/batch/" rel="nofollow">http://adamwhitcroft.com/batch/</a>