As a designer, I can’t overstate the value of a good flat, sharp, detailed icon. You can style it any way you please, use it in a variety of projects, and communicate difficult interface concepts with it.
iconsDB is an online place with a growing collection of free, flat, one color icons for designers to build amazing interfaces with. Currently, it has 2196 icons in the database that you can customize and download in any color and any size you want!
All icons can be used freely in both personal and commercial projects with no attribution.