Feather is a nice set of plug-and-play, open-source SVG icons.
The Noun Project is a vast collection of icons constantly updated by designers across the world. They now have a Mac App which allows for easy integration with all your favorite applications (Adobe CC, Sketch, PowerPoint, Messages, Mail, etc.).
Search over 100,000 icons. The Mac App constantly updates, giving you fresh content every day. ~ The Noun Project for Mac
Flaticon.com is a fabulous resource for finding all sorts of icons. Looking for vectors to use with your social media icons? They’ve got them.
Zion, named after my favorite national park, is an icon set designed to mesh with OS X Yosemite’s refined visual language. In Yosemite, toolbars have been simplified, favoring solid-colored circles for close, minimize, and maximize; windows and content feel lighter; and many frivolous elements like drop shadows and gloss have been scrapped. However, many of Apple’s own icons fail to reflect these system-wide UI changes, instead clinging to the realistic renderings of apps’ physical counterparts. Zion is meant to address these omissions, instill a system-wide consistency, and, most importantly, make our docks, folders, and desktops look a little more friendly.
~Alex Griendling, alexlikesdesign.com