Creative Market offers designs from independent designers and creative artists. Their SimpleLicense gives you the right to use any item purchased on Creative Market in a personal or commercial project for yourself or a client.

Batch - A free suite of 300 icons for web and user interface design. Icons by Adam Whitcroft.

Glyphish - 200+ grayscale icons by Joseph Wain. Free 1x size versions.

Fonthead Design's ClickBits and InfoBits fonts work well for Tab Bar and Toolbar icons. (Be sure to check out Fonthead's other great fonts, too!)

ikonic - 150 vector icons for iOS projects.

IconDock has several sets of color and grayscale icons.