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

iPhone UI Icon Set - a nice set of icons by designer Eddie Wilson. These are black and white icons ideal for tab bars, toolbars, and navigation bar buttons. A vector set is also available.

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!)

IconDock has several sets of color and grayscale icons.

Need a custom icon or design for your app? Create a design contest on 99 Designs and get a high-quality design for an affordable price.