The Doodlekit Icons set contains 700+ hand-drawn icons for use in sketch-style wireframes. You can buy them in vector format to use in the design tool of your choice, or download the free set of 32 x 32 px PNGs.

Here's a screenshot of the icons in the new custom icon feature that's available in the pre-release version of Mockups.

More info here.

http://doodlekit.imagiag.com