Here are five sites that will help you discover free icons to download.
Iconfinder is an icon search engine. What makes Iconfinder particularly noteworthy is it allows you to download just the individual icons in a set that you need. Note that Iconfinder also displays icons that aren’t free, but you can use the site’s "Free" filter to limit your search results.
iconmonstr has hundreds of free fonts available in PNG or SVG format. The icons are organized well in groups like "Business" and "Interface".
Fontello is a wonderful resource for web/UI designers looking for icon fonts. The site displays the icons of popular icon fonts like Font Awesome, Iconic, Brandico, etc. You can pick and choose just the icons you need, and when you’re done, it’ll combine them into one font file.
iconmelon has plenty of icons in SVG format, which makes this site a great bookmark-worthy resource for web/UI designers.
This site is an excellent vector icons search engine. It’s worth highlighting that Flaticon has a free Photoshop plugin which will help you find and use icons without needing to leave the Photoshop workspace.