I've been trying to come up with something that would make it convenient to do pixel-perfect screen layout while developing iOS apps. Something that would make it easy and visual to compare the mockup and the result. Today I hacked together this tool for OS X - an almost transparent window where you can open a mockup, place it over Xcode or the simulator, and see what you got. You can adjust the transparency with a slider to check how closely it matches.
Maybe it will be useful to someone. Or maybe someone else feels the same pain and will join the development.
The source code is on github.
P.S. my first app for OS X.